How To Stop A Fish Tank From Smelling – 7 Helpful Tips

Fish make excellent pets for so many reasons. These little wet pets are beautiful to watch and can have quirky little personalities. However, if you’re wondering how to stop a fish tank from smelling, it’s probably not so pleasant and serene anymore. It can be downright nasty, in fact. And who’s got to stick their hand into that water? It’s probably you, isn’t it?

You can stop a fish tank from smelling by cleaning and maintaining it regularly. Foul odors in fish tanks are caused by decaying matter in the water. A buildup of decaying organic matter causes a nitrate imbalance and an ammonia spike in the water, leading to a smelly fish tank.

What Makes A Fish Tank Smell Bad?

The primary reason a fish tank becomes smelly is because of decomposing organic matter. When organic matter decomposes, it causes a spike in nitrates and ammonia levels in the fish tank. Too much of it in a fish tank will cause the smell to become rather unpleasant. But what is decomposing matter, and why is it in your fish tank?

Apart from containing fish, a fish tank will host micro-organisms like bacteria, parasites, and maybe some plant matter. The waste products from the fish and micro-organisms are organic matter. Uneaten fish food is organic and starts to rot and stink like most food. Also, natural plants are organic, meaning they can decompose too.

The decomposing matter that can cause a foul odor in your fish tank can come from the following:

Poop Will Make A Fish Tank Smell

Fish, bacteria, and parasites all poop. Their waste goes into the water of the fish tank and settles at the bottom. If their poop is left for too long, it starts to decompose. It produces nitrates and causes a spike in ammonia. Not only does it smell awful, but it is harmful to your fish as it will lower their immune systems.

Uneaten Fish Food Will Make A Fish Tank Smell

As mentioned, fish food is organic and decomposes when left out in the open. If you overfeed your fish or aren’t eating well, the uneaten food will sink to the bottom and rot. Rotting fish food will contaminate the water and cause it to smell. Additionally, be careful not to spill fish food around the tank when feeding, as that, too, will begin to rot and stink up the room.

Dead Fish And Plants Will Make A Fish Tank Smell

Sometimes you might not notice that your little fish has passed on to fishy heaven. If left to decompose in the tank, its body will undoubtedly cause the tank water to smell. Additionally, if a plant starts to rot, it will also contribute to the smelliness of the tank.

7 Ways For How To Stop A Fish Tank From Smelling

There are quick and not-so-quick ways to stop the foul odor from your fish tank. Ideally, it would be best to familiarize yourself with the causes of smelly fish tanks, but we’ll get to that soon. For now, here are the ways to keep your fish tank from smelling awful:

1. Routine Fish Tank Maintenance Is Crucial

Keeping an aquarium is not a hobby for a lazy person. A fish tank requires frequent cleaning to ensure the health of your fish. Depending on your fish tank size and how many fish and plants you have, you should perform maintenance at least once a month. However, more frequent maintenance is recommended.

Here are some tips on maintaining and cleaning your fish tanks:

  • When changing the tank’s water, only replace 15 to 25% of the water at a time.
  • If you use tap water, you need to dechlorinate it before adding it to the fish tank.
  • Use a gravel vacuum to siphon water, uneaten food, debris, and waste from the bottom of the tank. The uneaten food, debris, and waste are organic and will start to decay and smell if left too long in the tank.
  • Replace the filter at least once a month, but not on the same day as your water change. If you do both on the same day, it can disrupt the biofilter media and good bacteria in the tank’s water, causing a stinky ammonia spike. So instead, alternate the times you change the water and replace the filter.
  • Avoid using tap water to clean your biofilter media, as chlorine will disrupt the bacteria. You can rinse it gently in removed tank water instead. Biofilter media can last for years without exposure to harsh chemicals.

2. Don’t Overfeed Your Fish

One of the biggest reasons for smelly fish tanks is that we overfeed our fish. Most of us are guilty of overfeeding our pet fish. They’ve got big, googly eyes and stare at you until you give in. But the truth is, fish don’t need a lot of food. The more fish eat, the more they’ll poop. More poop means more nitrates which will make the water stink.

A rule of thumb for feeding your fish is to feed them an amount they can consume in a few minutes. Also, sprinkle the food over a space of a few minutes. This will prevent the food from sinking to the bottom too fast.

3. Use Activated Carbon For A Quick Fix

If your fish tank has started to smell, but you haven’t the time to clean it that day, you can use activated carbon as a quick fix. Activated carbon in your fish tank filter will remove impurities from the water that cause a foul odor. Additionally, it will remove medications and cloudiness in your tank, leaving it crystal clear.

However, activated carbon should only be left in a filter for a short period. It is a chemical compound that leeches out its nasty impurities if left in water for too long. As such, you should replace the activated carbon in your filter at least once a month.

4. Monitor Oxygen, pH, And Temperature

To prevent a fish tank from smelling, it is wise to regularly monitor the oxygen, pH, and temperature levels. 


A low oxygen level in the fish tank leads to unhealthy fish. Their immunity will lower, and they will be prone to falling ill or dying, causing “water pollution” in the tank. Additionally, algae thrive in low-oxygen environments. Too much algae can also cause a smelly fish tank.

To maintain oxygen levels in your fish tank, you should ensure you have a wave maker, bubbler, or air stone. However, a bubbler and airstone might not be sufficient if you add more fish. More fish means less oxygen and more nitrates, which leads to (you guessed it) a smelly fish tank.


High pH levels can cause algae to bloom in the tank, which means more organic matter that can decompose and smell. The ideal pH of a fish tank is 7.0 or neutral. If the pH is below 7.0, the water is considered acidic. Above 7.0 means the water is alkaline. While fish can adapt to slight changes in the water pH, extreme pH levels will compromise the water quality and affect the fish. 


A fish tank that is too warm will cause algae to bloom or even cook your fish (let’s avoid that, shall we). Algae blooms and dead fish are a nasty combination that can cause your fish tank to smell. 

If you have a heater in your fish tank, be sure the thermostat works properly and keeps the water temperature consistent. You should ideally have a battery backup system for your heater and thermostat should the power fail. 

5. Don’t Leave On The Tank Light

Leaving the light on in a fish tank for too long can increase the temperature of the water and harm your fish. The light in your fish tank is more for your benefit than the fish’s benefit. Additionally, it’s not ideal for the fish’s health to always be in the light. If they were in the wild, they would have nighttime too. So, if possible, you shouldn’t keep your tank light on for more than eight hours a day.

6. Remove Dead Organic Matter Quickly

Most of us don’t want to see our pet fish doing the death bob. However, if that fateful day arrives, it’s best to remove Bobby’s corpse before it starts decomposing. As much as you loved him, he’ll stink up your tank if you leave him there.

There could be other unfortunate times when a fish was eaten by another fish in your tank. Some uneaten remains might be stuck in the gravel or floating around menacingly. That, too, will cause a smell. 

Other dead organic matter to remove includes live plants that are dying or pieces that have broken off. Overplanted tanks are more susceptible to stinking, so it’s best to have fewer plants.

How To Stop A Fish Tank From Smelling

7. Prepare Your Fish Tank Well Before Introducing Fish

You might be excited to start your new fish-keeping hobby, but you need to do some homework before plonking a fish into a new tank.

Whether you have a new or used fish tank, you should thoroughly clean it and let it run a cycle before introducing fish. Cleaning the tank well and rinsing it can help to control unwanted parasites and cleans off dust and chemicals.

But why should you run a cycle before introducing fish to a tank? Running your fish tank for a cycle allows the biofilter media to settle into their new “home.” If you add new fish, new water, and new biofilter media all at once, it will unsettle the nitrogen cycle in the tank and cause an ammonia spike. Not only is an ammonia spike smelly, but it is also dangerous to your fish.

Closing Thoughts

Hopefully these 10 strategies will help you prevent your fish tank from smelling. A little regular tank cleaning and maintenance will definitely help avoid any unpleasant aquarium odors.

You may also be interested in reading How To Get Crystal Clear Aquarium Water.

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