17 Signs Of Low Oxygen In Aquarium [Reasons and What To Do]

Aquariums bring the underwater world right into our homes, filled with colorful fish and interesting creatures.

But did you know that just like us, fish need oxygen to live and be happy? It’s important to make sure they have enough of it.

Have you ever wondered if your aquarium buddies are getting the oxygen they need, especially since different fish need different amounts?

This article will show you 17 signs of low oxygen in aquarium and what you can do to help your fish breathe better.

Let’s dive in and find out how to keep our underwater friends healthy and lively!

17 Signs Of Low Oxygen In Aquarium

In this article, we’re going to show you 17 important signs that your fish tank might not have enough oxygen.

We’ll look at how your fish act, what the water looks like, and the condition of the tank itself to figure out if there’s a problem.

And don’t worry, we won’t just tell you about the problems!

For each sign, we’ll also share some helpful tips on how to fix it and ways to stop it from happening again.

Signs Of Low Oxygen In Aquarium - Fish Behavior Water and Tank Conditions

So, let’s learn together how to keep our fish friends happy and healthy!

Fish Behavior Signs

In the very first section, we’ll explore how fish act differently when they’re not getting enough oxygen.

You’ll learn to spot unusual behaviors like fish gasping for air at the top of the tank or not swimming around much.

These clues can help you figure out if your fish need more oxygen, so you can help them feel better!

Labored Breathing

When fish don’t get enough oxygen, they breathe hard and fast, almost like they’re trying too hard to breathe.

This happens because there’s not enough oxygen in the water for them to breathe in easily. You might see their gills moving very fast.

This can happen any time the water doesn’t have enough oxygen, maybe because the tank is too crowded or the water is dirty.

Preventive Measure

To help fish with labored breathing, you can add an air pump or air stone to the tank.

This adds more oxygen to the water, making it easier for the fish to breathe.

Also, make sure the tank isn’t too crowded and keep the water clean.

Gasping at The Surface

Fish gasp at the surface when they’re desperate for air because the water below doesn’t have enough oxygen.

They do this to try and get oxygen directly from the air above the water.

This often happens when the water is too warm or hasn’t been changed in a while.

Fish Gasping at The Surface

Preventive Measure

Adding an air pump to the tank can increase the oxygen level.

Also, keeping the water cool and fresh by doing regular water changes helps prevent fish from gasping for air.

Reduced Activity

When there’s not enough oxygen, fish become less active and swim around less.

They do this to save energy because moving too much requires more oxygen.

This can happen if the tank is too small or if there are too many fish using up the oxygen.

Preventive Measure

Make sure your tank is big enough for all your fish and not overcrowded.

Increasing the oxygen with an air pump or water filter can also help make them more active.


Fish become very slow and may not move much at all when they’re lethargic.

This is because they don’t have enough oxygen to be active.

Lethargy often occurs in polluted water or when the water temperature is too high.

Preventive Measure

Keep the water clean and at the right temperature.

Using a water filter and changing the water regularly can help prevent lethargy by keeping oxygen levels high.

Erratic Swimming

When fish don’t get enough oxygen, they might swim in strange ways, like darting around quickly or flipping over.

This happens because they’re stressed and trying to find more oxygen.

It can happen in dirty or very warm water.

Preventive Measure

Make sure your tank is clean and not too warm.

Adding a water filter or air pump can help stabilize the oxygen levels, making fish swim normally.

Decreased Appetite

Fish may not eat much or at all if there’s not enough oxygen.

They do this because their bodies are trying to save energy, and eating requires energy.

This can happen if the water is stale or if there are too many fish in the tank.

Preventive Measure

Improving the oxygen in the tank can make fish feel better and more interested in eating.

You can do this by adding an air stone or making sure your water filter is working well.

Also, don’t overcrowd your tank.

Other fish signs guide you must read:

11 Signs Of a Stressed Betta Fish [and How To Calm Them]

7 Abnormal Betta Fish Behavior You Need To Know

Water Characteristics

In this section, we’ll dive into how the water in your fish tank can tell you about oxygen levels.

If the water looks strange, like it’s not moving much or it’s changing color, it might mean there’s not enough oxygen.

Spotting these water signs early can help you keep your tank healthy and your fish happy!

Temperature Fluctuations

When the temperature in your fish tank goes up and down a lot, it can be a sign that your water isn’t healthy.

This happens because when water gets too warm, it can’t hold as much air.

Fish need this air to breathe.

If the water changes temperature quickly, it might mean your fish aren’t getting the air they need.

Temperature Fluctuations Of Tank Water

Preventive Measure

Keep your tank away from sunlight and use a water heater or cooler to keep the temperature steady.

This helps make sure your fish have enough air to breathe and stay healthy.


If the water in your tank isn’t moving, it might look still or have a film on top.

This stillness means the water isn’t getting enough fresh air mixed in.

When water doesn’t move, it doesn’t let in new air from the surface, which fish need to breathe.

Preventive Measure

Adding a filter or an air pump to your tank can help move the water around.

This brings in fresh air and keeps your fish happy and healthy.

Poor Quality

Water that’s turning cloudy or getting dirty can be a sign that it’s not healthy.

This happens when waste from the fish and leftover food builds up.

This waste can use up the air in the water, leaving less for the fish.

Preventive Measure

Regularly cleaning your tank and changing some of the water can help keep it clear and full of air.

Using a water filter also helps keep the water clean and rich in air for your fish.

Tank Conditions

In the last section, we’re going to check out what clues your fish tank gives when oxygen is low.

Things like too many fish in a small space or dirt and plants taking over can mean trouble.

Learning these signs helps you fix problems fast and keeps your fish swimming.


Over Crowding

When there are too many fish living in a small tank, it can lead to not having enough air for all of them.

This happens because each fish needs a certain amount of air to stay healthy, and if there are too many fish, they use up the air faster than they can be replaced.

This problem can pop up when new fish are added without making sure there’s enough space and air for everyone.

Preventive Measure

To fix this, make sure you have a tank that’s big enough for the number of fish you want.

Sometimes, moving some fish to another tank or getting a larger tank can help make sure there’s enough air for everyone.

Excessive Algae Growth

Lots of algae in your tank can be a sign that your water isn’t as healthy as it should be.

Algae use up oxygen in the water, especially at night, which can leave less for your fish.

This usually happens when there’s too much light or leftover food in the tank.

Preventive Measure

Cutting down on the amount of light your tank gets and cleaning out uneaten food can help control algae growth.

Adding plants that compete with algae for nutrients and light can also help keep your tank’s air levels healthy.

Organic Decay

When plants or fish waste start to rot in your tank, it can use up a lot of air.

This decay process needs oxygen, and it can take it away from your fish.

You might notice this happening if your tank starts to smell bad or the water gets cloudy.

Preventive Measure

Regular cleaning and removing any dead plants or uneaten food can help stop organic matter from decaying in your tank.

Using a good filter can also keep the water clean and full of air.

Inadequate Filtration

Inadequate Filtration

A filter that’s not working well might not clean the water properly, leading to low air levels.

Filters help by moving the water around and adding air back into it. If the filter is too small or not working right, it can’t do its job.

Preventive Measure

Check your filter to make sure it’s the right size for your tank and that it’s working correctly.

Sometimes, cleaning the filter or replacing it with a more powerful one can help improve the air in your tank.


Seeing a lot of water droplets on the inside of your tank cover can mean the air inside your tank is not moving well.

This still air can’t hold much oxygen, which is bad for your fish.

Condensation can happen if the tank is too warm or if there’s not enough airflow.

Preventive Measure

Improving airflow in and around your tank can help.

Make sure there’s enough space around your tank for air to move freely, and consider using a tank cover that allows air to get in and out more easily.

Additional Observations

Rare and Uncommon Signs Of Low Oxygen In Aquarium

In this section, we’ll explore some unusual hints that your aquarium might have low oxygen, things you might not expect!

Like strange bubbles or fish acting in ways you’ve never seen before.

Spotting these rare signs can help you keep your fish tank perfect and safe.

Chemical Interference

Certain chemicals used for treating water conditions or diseases might inadvertently reduce the water’s capacity to hold dissolved oxygen.

This effect is not always immediately noticeable and can depend on the dosage and combination of chemicals used.

Biofilm Overgrowth

While some biofilm is normal and even beneficial in a tank, excessive biofilm development, especially on the water surface, can significantly reduce gas exchange.

This rare scenario might occur in tanks with very stagnant water or extremely high nutrient levels.

Plant Overgrowth

In heavily planted tanks, plants consume oxygen during the night (respiration) while not photosynthesizing (which produces oxygen).

An imbalance in plant mass and tank size, especially without adequate aeration, can lead to overnight oxygen depletion.

Decomposition of Large Plant or Animal Matter

If a large fish dies unnoticed or if there is significant plant decay, the decomposition process can consume a lot of oxygen, more than what’s commonly expected from the usual bioload.

Algal Blooms

Algal Bloom

While algae are known to produce oxygen during the day, intense and sudden algal blooms can deplete oxygen levels at night through respiration, competing with aquatic animals.

This scenario is more common in nutrient-rich environments with strong light exposure but is often overlooked as a cause of low oxygen.

Inadequate Circulation in Deep or Complex Tanks

In tanks with complex layouts or significant depth, water circulation might be insufficient to distribute oxygen evenly.

This can create zones with lower oxygen levels, a situation often missed during standard checks.

Use of Certain Substrates

Some aquarium substrates, especially those designed to alter water chemistry (like reducing pH), can also affect oxygen levels.

This interaction is complex and can vary depending on the specific materials and tank conditions.

Sudden Atmospheric Pressure Changes

Rarely, sudden and significant changes in atmospheric pressure can affect the gas equilibrium in aquarium water, leading to temporary disruptions in oxygen solubility.

This situation is usually transient and resolves as conditions stabilize.

Heavy Metal Contamination

The presence of heavy metals in the water, either from contaminated water sources, certain types of decor, or accidental inclusion, can impact the respiratory efficiency of aquatic animals, effectively reducing the available oxygen for their use.

Importance Of Maintaining Oxygen Levels

Importance Of Maintaining Oxygen Levels

Maintaining oxygen levels in your aquarium is key for keeping your fish healthy and your tank clean. Low oxygen can harm fish and upset the tank’s balance.

Low Oxygen Level Can Be Detrimental To Fish

When there’s not enough oxygen in the water, fish can get sick.

They need oxygen to breathe, just like we need air. Without enough oxygen, fish can become stressed and might not survive.

Leading Fish To Distress Or Death

Fish that don’t get enough oxygen can start to act oddly, like swimming near the water’s surface where there’s more oxygen or moving their gills fast.

If it gets too bad, they could even die.

Promotes Healthy Fish Metabolism

Oxygen helps fish turn their food into energy in a process called metabolism. When there’s plenty of oxygen, fish can grow strong and healthy because their bodies are working well.

Ensures the Stability of the Aquarium Ecosystem

A tank with the right amount of oxygen is a happy place for all living things in it. It helps plants grow, keeps the water clean, and makes sure everything is balanced.

Prevents Algal Overgrowth and Maintains Water Quality

Too much algae can be a problem, and it grows fast when there’s not enough oxygen. Keeping oxygen levels up helps stop algae from taking over and keeps the water nice and clear.

Helpful article:

Can Too Much Oxygen In Aquarium Causes Algae? [Read These 7 Reasons]

How To Get Rid Of Black Algae In Fish Tank? [5 Proven Tips]

Step-by-Step Guide to Oxygen Testing in Your Aquarium

Next, we’ll walk through a 7-step guide on how to test the oxygen levels in your aquarium. It’s like a science experiment for your fish tank!

For each step, we’ve got a fun picture to show you exactly what to do.

From collecting water to checking results, these steps will help you make sure your aquarium has enough oxygen for your fish to be happy and healthy.

Let’s dive in and learn how to do this important test!

Step-by-Step Guide to Oxygen Testing in Your Aquarium

Step 1: Gather Your Tools

Tools Needed: A dissolved oxygen test kit.

Step 2: Collect a Water Sample

Fill the test tube from your kit with aquarium water.

Step 3: Add the Reagent

Carefully drop the reagent (a chemical from your kit that reacts with oxygen) into the test tube as instructed by your kit’s manual.

Step 4: Shake Gently

Cap the test tube and gently shake it to mix the water with the reagent.

Step 5: Wait for the Reaction

Let the test tube sit for the amount of time specified in your kit’s instructions.

Step 6: Check the Color Change

Compare the color of the water in your test tube to the color chart provided with your kit. This will tell you the oxygen level.

Step 7: Take Action

If the oxygen level is below the safe range (6-8 mg/L for most fish), follow the steps in our guide to increase oxygen in your tank.

Read this next:

How To Add Oxygen To Fish Bowl? [Try My Personal 5 Ways]

Emergency Measures (That I Suggested My Friends!)

When your fish start gasping for air, it’s all hands on deck!

I’ve shared these life-saving tips with my friends to keep their aquatic pals happy and breathing easy.

Let’s dive into these emergency action tips for oxygen boosts!

Emergency Measures When There Is Low Oxygen In Fish Tank

Performing a Large Water Change

Imagine your fish wearing tiny snorkels, but there’s not enough air.

What do you do? Swap out the stale water!

A big water change is like opening a window for fresh air.

It washes away the old, yucky water and brings in a breath of fresh life, giving your fish a much-needed oxygen boost.

How This Tip Helped My Friend Abhijit

When my friend Abhijit’s fish started acting like they were gasping for their last breath, he was as worried as a long-tailed cat in a room full of rocking chairs. That’s when I threw him the lifeline of a big water change suggestion. Skeptical but desperate, Abhijit swapped out a hefty portion of the tank’s stale water for fresh, clean water.

To his amazement, it was like flipping a switch. His fish went from lethargic and near-death to zipping around the tank with the zest of kids on a sugar rush. Afterward, Abhijit couldn’t stop thanking me, saying, “I thought my fish were history, but your water change tip was a game-changer. You’re the fish whisperer, my friend!”

That day, a simple act of changing water breathed new life into Abhijit’s aquarium, turning despair into delight.

Enhancing Water Movements With Powerheads

Fish tanks can be too calm. It’s like a party without music!

Adding powerheads stirs things up, making waves that bring oxygen into every corner of the tank.

It’s like turning on the fans at a hot, stuffy room party—suddenly, everyone can breathe and dance again.

Use Air Stones To Generate Oxygen

Use Air Stones To Generate Oxygen

Air stones are the fizzy tablets of the fish world.

Pop one in, and watch it bubble up like a soda!

These bubbles aren’t just for show; they’re a lifeline of oxygen for your gilled friends.

It’s like adding sparkle to water, making it irresistible to breathe.

Use Oxygen Filters

Oxygen filters are the superheroes of the aquarium.

They swoop in, sucking up dirty water and blasting out clean, oxygen-rich water.

It’s as if you had a personal cleaner who not only tidies up but also plants little oxygen gardens throughout your home.

Add Oxygen Tablets

Oxygen tablets are like mints for your aquarium.

They freshen up the place instantly! Drop one in, and it dissolves, releasing precious oxygen bubbles.

It’s a quick fix, like popping a breath mint before greeting your friends with a big, fresh smile.

Here’s How Oxygen Tablets Helped My Friend Jasmine

Jasmine was facing a crisis with her aquarium; her fish was as listless as a lazy afternoon. I mentioned oxygen tablets could perk things up. Initially, Jasmine was stressed — no tablets at hand and a tankful of sluggish fish. However, recalling an aquarium shop nearby sparked a glimmer of hope. She made a quick trip, snagging the tablets she needed.

Back home, with a mix of hope and skepticism, she dropped the tablets into the tank. To her relief and surprise, the water soon fizzed with oxygen, reviving her fish. “I was worried those tablets wouldn’t make a difference, but it was like watching my tank come back to life,” Jasmine shared, amazed. Her aquarium went from stagnant to spirited, all thanks to a simple, yet effective solution

Adding Cold Water

Pouring cold water into your tank is like splashing your face with water in the morning—it wakes everyone up!

Colder water holds more oxygen, so it’s a chilly but effective way to give your fish a pep talk.

Just remember, no ice cubes—you don’t want to shock your aquatic buddies into thinking winter came early!

The Temporary and Permanent Solutions

Now, we’re diving into solutions to boost your aquarium’s oxygen levels.

First, we’ll explore temporary fixes that are quick and easy to apply.

These are super helpful in urgent situations but think of them as a band-aid solution.

For a healthy, thriving aquarium in the long run, we’ll also talk about permanent solutions.

Remember, it’s cool to start with quick fixes, but for your fishy friends’ sake, aiming for long-lasting solutions is the way to go.

Temporary and Permanent Solutions For Low Oxygen Levels In Aquarium

Temporary Solutions

After learning about emergency measures to tackle low oxygen, it’s time to look at some temporary solutions.

These are like the first-aid kit for your aquarium’s oxygen problems. They work fast and can help out in a pinch.

But just like putting a plaster on a scrape, they’re not the end-all solution.

Let’s dive into some of these quick fixes and see how they can help for now.

Adding Ice Cubes

Do you have a hot tank? Cooling down your aquarium water with ice cubes can be a quick fix.

Cold water holds more oxygen, so this can give your fish a quick breath of fresh air.

Use this trick when your tank’s temperature spikes, especially on hot days.

Just remember to wrap the ice in a bag so it doesn’t mess with your water balance!

Battery Powered Air Pumps

Battery Powered Air Pumps

When the power’s out or your main pump fails, a battery-powered air pump is a lifesaver.

It keeps the water moving and adds oxygen, keeping your fish happy until things are back to normal.

It’s great for emergencies or when you’re moving your tank to a new spot. Always have one handy, just in case!

Pouring Water From a Height

Pouring water back into your tank from a height can mix things up and add oxygen quickly.

It’s like making tiny waterfalls inside your tank!

Do this when you notice your fish gasping for air at the surface.

It’s a quick fix to help them catch their breath but remember, it’s just for now.

Stirring The Water Manually

Sometimes, just stirring the water can add a bit of extra oxygen.

Use a clean spoon or stick to gently swirl the water around.

It’s a simple move, but it can make a big difference when your fish needs oxygen ASAP.

Think of it as giving your tank a little pep talk to get through a tough time.

Permanent Solutions

Now that we’ve tackled temporary fixes for low oxygen in your aquarium, it’s time to level up to permanent solutions.

These are the real deal, ensuring your water world stays oxygen-rich and vibrant for the long run.

By investing a little more time and effort now, you’re setting up your aquatic paradise for success.

No more quick fixes; these solutions are all about keeping your tank’s ecosystem stable and your fish flourishing forever.

Installation Of Hang-On-Back Filters

Hang-on-back (HOB) filters are not just space savers; they’re oxygen boosters too.

By pulling water up and splashing it back into the tank, they help mix in loads of oxygen.

Perfect for any tank size, they’re an easy-to-maintain solution for a happier, healthier aquarium.

Spray Bars

Spray bars spread water evenly across the surface, creating more space for oxygen to mix in.

By adjusting them to create a gentle ripple, you ensure your fish gets a steady supply of oxygen without creating a stressful current.

It’s like giving your tank a breath of fresh air!

Air Pumps

Air pumps are the heart of oxygenation, pumping air into your water through stones or decorations.

This constant flow keeps oxygen levels up and ensures your fish have plenty of air to breathe.

It’s a must-have for any tank, especially those with a lot of aquatic friends.

Dissolved Oxygen Meters

Keep track of oxygen levels with a dissolved oxygen meter.

It’s like having a health monitor for your tank, letting you know when oxygen is low so you can act fast.

By keeping an eye on oxygen, you’re ensuring your fish aren’t just surviving but thriving.

Oxygen Reactors

Oxygen reactors are next-level tech for serious aquarium enthusiasts.

They inject oxygen directly into the water, making sure there’s always enough to go around.

If you have a large tank or lots of fish, an oxygen reactor could be your tank’s new best friend.

Venturi System

The Venturi system mixes air with water as it pumps back into the tank, creating a super-oxygenated environment.

It’s a slick way to boost oxygen without adding extra equipment.

Perfect for any tank setup looking for a breath of fresh air.

Proper Tank Stocking

Keeping the right number of fish is key to maintaining oxygen levels.

Overcrowding means more mouths to breathe, depleting oxygen fast.

Stick to the recommended stocking levels for your tank size to keep everyone happy and healthy.

Live Plants

Live Plants

Live plants are nature’s oxygen generators, photosynthesizing light into precious O2.

They not only add oxygen but also consume CO2, keeping your tank’s ecosystem balanced.

Plus, they make your aquarium look amazing!

Use Of Right Chemicals For Aquarium Water

Using the right chemicals ensures your water’s quality supports oxygen absorption.

Some additives can increase oxygen levels, while others make sure the water’s just right for oxygen to dissolve efficiently.

It’s about creating the perfect home for your aquatic buddies.

By implementing these permanent solutions, you’re not just solving the oxygen problem; you’re transforming your aquarium into a self-sustaining ecosystem. Your fish will thank you with bright colors, active swimming, and overall better health. Dive into these solutions and watch your aquatic world thrive like never before!


And there you have it, folks!

Keeping your underwater buddies bubbly and breathing isn’t just a walk in the park. It’s an adventure!

Let’s quickly bubble up the big things we learned:

  • Watch for Signs: Remember, if your fish are gasping or being lazy, they’re telling you they need more oxygen.
  • Cool Water Rocks: Cold water is like a magic potion for oxygen. Keep it cool to keep them happy.
  • Bubble It Up: Air pumps and air stones aren’t just fun to watch; they’re lifesavers for your fish.
  • Clean House: A clean tank means happy, healthy fish. Don’t let dirt and algae crash the party.
  • Space Matters: Don’t crowd your fish. They need room to swim and breathe!

In short, keeping an eye on your aquarium and giving it a little care goes a long way.

Treat your fish like the treasures they are, and they’ll keep the underwater show going strong.

Dive into these tips, and watch your aquarium thrive!

Recommended article:

9 Signs Of Too Much Oxygen In a Fish Tank

Detailed Guide On Managing Aquarium Oxygen Levels

15 Signs Of Too Much CO2 In Aquarium [Causes + 5 Solutions]

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