Why Does My Fish Tank Smell Bad? Main Reasons & Solutions

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A stinky aquarium can be pretty unpleasant, especially if it’s located in an area you frequent the most like your living room. You might have wondered “why does my fish tank smell?” and whether there’s anything you can do to solve the problem.

The most common reasons why your fish tank may smell are rotting fish, leftover food, fish poop, dead plants, dirty substrate, and an unclean filtration system. Frequent water changes and removing the cause of the bad smell will help you fix the issue.

If your aquarium has a horrible smell that won’t go away, our guide will help you figure out the cause and the best way to treat it.

Why Does My Fish Tank Smell Bad? Main Reasons

All aquariums have a slight odor, but a healthy fish tank should not have a particularly strong or foul scent. If your setup has an overpowering stench that’s earthy, “fishy”, sulfurous, or just downright funky, it usually indicates a problem.

Some of the main reasons why fish tanks can develop a bad smell include:

Dead fish
Uneaten fish food
Fish waste
Decaying plants
Dirty filter
Water conditioners
Algae

This video does a great job explaining some of the causes of a smelly aquarium:

Dead Fish

Sadly, one of the most common causes of foul smells in aquariums is dead fish. When a fish dies, their body rapidly decomposes and leeches oil/fat into the water. In saltwater and freshwater tanks with warm water, decomposition takes place much sooner.

A decaying fish not only smells bad, but it can also lead to surface film in your tank. Unfortunately, you may not notice that one of your fish has passed away for several days, particularly if they are small or die behind aquarium décor.

It’s always a good idea to check your aquarium at least once a day for missing fish. Look behind filters, plants, decorations, and any other places a dead fish may have become lodged.

If your fish die, remove the body as soon as possible and immediately perform a water change. Decaying fish can lead to elevated ammonia levels in the aquarium, more so if they have been dead for quite some time.

A tank with high ammonia levels can kill your fish and may require you to undergo the cycling process again.

Excess Food

While it’s important to feed your fish enough to keep them healthy, overfeeding can be just as harmful as underfeeding. Giving your fish too much food can lead to health issues like constipation, swim bladder disease, and a fatty liver.

Excess food can also wreak havoc on your water quality and contribute to bad aquarium smells. All the food that your fish don’t eat will fall to the bottom of the tank, which encourages the growth of anaerobic bacteria.

As the bacterial colonies expand, they release foul-smelling gasses that give your tank a sulfurous odor (like rotting eggs).

Fish Poop

A high amount of fish poop in your tank can cause bad smells and elevated ammonia levels. This is common in overstocked aquariums or small setups that contain large species of fish.

While your filter and the good bacteria in your aquarium can help process some of this waste, they can’t remove all of it. It’s important to follow a regular tank maintenance schedule and make sure your setup is big enough for the number of fish it contains.

Decomposing Plants

Another common cause of smelly aquarium water is rotting plants. Dead plants can make your fish tank cloudy and give it a very bad smell if they are left to decay. It’s easy to check whether you have a rotting plant as it will turn brown and have a slimy coating.

Make sure you remove dead plants and any decaying plant leaves as quickly as possible.

Dirty Filtration System

Although filters can help clean your water, they need regular maintenance to keep them in tip-top shape. All that debris and waste that your filtration system sucks up will need to be removed, otherwise, it will make your fish tank smell bad.

You’ll know if your filter needs cleaning when you open it up. Not only will it smell foul, but there will be a build-up of brown sludge inside the chamber and on the filter media. That gunk is a mixture of fish poop, uneaten food, and dead plant life, so it can have a pretty strong stench!

Aquarium Water Conditioner

Water conditioners are essential for removing chlorine, chloramine, and heavy metals from your aquarium water, but some types can have an unpleasant odor. The bad smell is most prominent when you first add water conditioner to your water, but it will eventually pass.

For example, Seachem Prime has a characteristic smell of rotten eggs. Despite its bad odor, it’s a fantastic water conditioner and can detoxify ammonia/nitrite for 24 hours, making it a great product to have on hand in times of emergency.

It also contains slime coat stimulants that can help improve your fish’s disease and chemical resistance.

Algae

An algae outbreak in your fish can smell bad, especially if it’s blue-green algae. Algae is usually caused by too much light, overfeeding your fish, and excess nutrients in your water.

Rotten algae can also smell bad, so you should make sure you remove it with a scraper or sponge straight away. Algae can cover every surface in your fish tank, including the substrate, glass, decorations, and plants.

How To Fix A Fish Tank That Smells Bad

Bad smells in an aquarium can make keeping fish less enjoyable as no one likes to be around a stinky tank. Besides, you want to see your fish, not smell them!

Fortunately, there are a few ways you can fix a fish tank that smells bad, but you’ll need some supplies first.

Supplies Needed

Below are all the supplies you need to fix a smelly fish tank.

Step-By-Step Removal Method

Removing bad smells from a fish tank can be achieved by giving your aquarium a deep clean. This includes vacuuming your substrate, wiping down your aquarium décor, cleaning your filter, and adding activated carbon to your filter.

The process can take a while (a few hours if your tank is very dirty), but you should eventually end up with a clean fish tank that is odor-free.

Step 1: Using an aquarium sponge or cleaning brush, remove all decorations from your tank and clean them in a bucket of old tank water. If there are dead plants or decaying fish in your aquarium, remove them with a net. Wipe down the tank glass to get rid of algae or debris.

Step 2: Vacuum your substrate using a gravel cleaner to remove excess food, poop, and other dirt that has fallen to the bottom of the tank. Don’t forget the corners!

Step 3: Turn off your aquarium filter and open it up. Using old tank water, remove debris from inside your filter and give your filter media a clean. Dunk the media in the water and squeeze it a few times. Never use tap water as it will kill the beneficial bacteria in your filter media.

Step 4: Clean the inflow and outlet of your filter system to dislodge any sludge. You should also make sure your impeller isn’t clogged.

Step 5: Put your filter system back together and place it back in your aquarium, then perform a partial water change on your tank (around 25%).

Step 6: Fill a bucket with clean water that has been treated with an aquarium water conditioner. Only use treated water as municipal tap water contains chlorine and chloramine, both of which are toxic to aquatic life.

Step 7: Add activated carbon filter media inside your clean filter. Place it after your mechanical filter media.

Step 8: Use an aquarium test kit to check your water parameters and make sure everything is as it should be. Ammonia and nitrite levels should be zero, while nitrate levels should be no higher than 20ppm.

How To Prevent A Fish Tank From Smelling Bad

The best way to remove bad tank smells is to stop them from happening in the first place! Using the right preventative measures will help you steer clear of smelly aquarium water for good.

Don’t Overfeed Your Fish

Fish are opportunistic feeders, so they will never say no to food. While it’s important to offer a variety of foods to keep your fish healthy, too much food will have a negative impact on their well-being and your water quality.

As mentioned earlier, too much food can cause bloating, digestive issues, and a fatty liver. Excess food can also lead to the growth of bacterial colonies in your substrate that can make your tank smell.

Only feed your fish as much food as they can eat in two or three minutes. Use a net to remove any leftover food to help keep your substrate and fish tank clean. Most fish only need feeding twice a day, so try not to offer more meals than this.

Don’t Overstock Your Fish Tank

As tempting as it can be to fill your tank with lots of different fish, you need to keep a close eye on your stocking level. It’s always better to understock a tank than overstock it.

Use the “one inch of fish per gallon of water” rule to help you calculate the number of fish your setup can hold. Remember to go by the adult size of your fish!

Tanks with too many fish will have high bio-loads, which can lead to poor water quality and elevated ammonia levels, as well as bad smells due to all the fish waste. It’s much harder to maintain water parameters and good water quality in overcrowded aquariums.

Remove Dead Fish Quickly

Unfortunately, dead fish is an inevitability as an aquarist. Although it’s never nice removing a decaying fish from your tank, it’s important to do it quickly to prevent water fouling.

A fish that has passed away will leak oils and proteins into the water, which can lead to surface scum, bad smells, and ammonia spikes. Check your aquarium for sick or dead fish every few hours so you can intervene rapidly.

Fish corpses can get stuck behind filters, plants, decorations, heaters, etc., so be sure to check these areas regularly. It’s also worth using a tank hood or cover if you have jumping fish like a betta to prevent them from leaping out of your aquarium to their death.

Trim And Prune Live Plants

Live plants can look stunning in a fish tank, but they don’t look quite as appealing once they’re decaying or overgrown. Rotten plants will produce bad tank smells and result in high nitrate levels, the latter of which can be harmful to aquatic life.

Trim off rotting leaves on your plants (they’ll look brown and slimy) to promote new growth and keep your fish tank clean. If there are any dead plants in your aquarium, remove them straight away.

Perform Frequent Water Changes

Water changes are an essential part of keeping fish to maintain stable water parameters and good water quality. Poor tank maintenance will lead to dirty water, foul odors, fluctuations in water chemistry, and stressed fish. It will also make them more likely to get sick.

Aim to do a partial (around 25%) water change once a week – make sure you use treated water to fill your aquarium back up. You may need to do water changes more often if your setup is overstocked or your fish produce a lot of waste.

Test the parameters of your fish tank water at least once a week with an aquarium water test kit.

Clean Your Fish Tank Substrate

Uneaten food, decaying plant matter, and fish feces can get trapped inside your substrate, which can lead to the growth of anaerobic bacteria. This is common in fine sand and gravel as they contain tiny pockets where debris can fall between.

Anaerobic bacteria can make your aquarium smell bad like rotten eggs and result in high ammonia levels, so you need to prevent it from growing by regularly vacuuming your substrate with a gravel cleaner.

Vacuuming the substrate will help you remove fish waste, fish food, and plant matter, as well as stop the formation of dead zones.

Clean Your Filter Regularly

Regular filter maintenance not only keeps your filtration system working properly, but it also prevents bad smells. Over time, filters can become clogged with brown sludge and other gunk that can have a stinky scent.

You should clean your filter every three to four weeks. Use old tank water when conducting maintenance on your aquarium equipment so you don’t kill off any beneficial bacteria.

If you’re replacing filter media, don’t throw it out all at once as you’ll also be destroying the good bacteria colonies that are needed for removing organic and nitrogenous wastes.

Use Activated Carbon

Activated carbon is a great way to remove bad fish tank smells as it effectively absorbs phenols (the causes of bad fish tank smell) in your water. Activated carbon also gets rid of tannins (discoloration), heavy metals, and chlorine.

Activated Caron is one of the best fixes to preventing and eliminating strong odors in fish tanks. You can purchase loose activated carbon or carbon filter cartridges online or from your local pet store.

Place the carbon inside your filter, ideally after your mechanical filter media.

Activated carbon loses its absorbency and efficacy over time, so you will need to replace it every two to four weeks. If your fish tank smells bad again, it’s probably time to change your carbon filter media!

Remove Or Reduce Algae

Algae blooms in your aquarium can cause a real stink and make your tank look unsightly. You should take steps to remove or reduce the amount of algae in your aquarium to keep your water healthy and odor-free.

Don’t leave your aquarium lights on for long periods (no more than 8 hours) and limit the number of nutrients in your water. Algae thrive in water with high phosphate levels, so feed your fish sparingly and perform regular water changes.

If your fish tank is positioned near a window that receives direct sunlight, it’s worth relocating it to a darker area. You can also remove algae manually using a scraper, sponge, or net.

Some algae can be killed with algae removal products containing copper sulfate or cupric sulfate.

Test Your Fish Tank Water Regularly

You should test your aquarium water with a test kit regularly to keep an eye on your parameters as this will alert you to any problems with your water. For instance, if you notice ammonia beginning to rise in your tank, it could be due to a dead fish or rotten plant.

Well-established aquariums should have their parameters tested every few days, but daily testing is recommended for uncycled or newly cycled aquariums as they are more prone to fluctuations in water chemistry.

What To Do If A Fish Tank Smells Bad Even After Cleaning It

While thoroughly cleaning your aquarium can help reduce unpleasant tank smells, there are times when this method is ineffective. So, what can you do if your fish tank smells bad even after cleaning it?

First, remove the cover or hood of your tank as there may be some waste or debris caught under the lid that you didn’t notice. Check the outside area around your fish tank for grime or dead fish (some fish species are known jumpers!).

If your fish tank hood and aquarium are clean, then the best way to remove stinky tank smells is by using activated carbon in your setup. Carbon is highly porous and absorbent, so it will quickly soak up foul odors in your fish tank.

Can A Smelly Fish Tank Harm Fish?

Although a smelly fish tank itself won’t harm fish, it can point to an underlying issue with your aquarium. A problem with your fish tank can certainly harm or even kill your fish if it isn’t addressed.

For instance, anaerobic bacteria caused by a dirty substrate can lead to bad water quality and elevated ammonia levels. Both of these can make your fish very sick and eventually lead to death.

You should never avoid bad fish tank smells – always try to identify the cause of the odor so you can fix it quickly.

Can A Smelly Fish Tank Make You Sick?

Bad fish tank smells won’t make you sick, but their presence can mean that there is something wrong with your aquarium. In addition, you need to take care when cleaning your aquarium to eliminate bad fish tank smells, especially if you have open wounds or cuts on your hands.

The Mycobacterium marinum thrives in non-chlorinated water. It has been known to cause infections in humans, including a pretty nasty disease called aquarium granuloma.

You can catch this infection by bites or injuries sustained by handling fish, but you’re most likely to contract it from cleaning a fish tank with broken skin.

The main symptom of a Mycobacterium marinum infection is skin lesions. These lesions can be sore or completely painless, and they tend to form in clusters on your hands/fingers.

Most cases of this disease can be treated with antibiotics, but you may need surgery if the infection is particularly severe.

Bear in mind that his type of infection is pretty rare, so it’s likely to be misdiagnosed as something else, such as cellulitis or a fungal infection.

Conclusion

Bad tank smells are a common issue for many aquarium owners, but hopefully, this guide helped you identify the cause of your stinky fish tank and the best way to fix it!

Remember, a healthy aquarium shouldn’t smell foul or unpleasant, so if your fish tank smells bad, there’s likely an issue with your water. Uneaten food, fish waste, decaying plants, and dead fish are some of the most common causes of foul odors in tanks.

You should clean your tank and substrate frequently, as well as remove dead fish or plants before they have a chance to decay. Don’t forget to use activated carbon on a regular basis to keep bad smells at bay!

Recommended Reading:

Frequently Asked Questions, Tank/Aquarium Guides