Is It Normal For A Fish Tank To Smell?

I am enjoying my fishes and suddenly I smell a bad odor. It’s a stinky smell and I recognize it’s from my fish tank. I am thinking for yourself is this normal for a fish tank?

So, is it normal for a fish tank to smell? It’s not normal for a fish tank to smell but there are different reasons for it to smell.

It’s a clear sign that something is up to something bad if the fish tank stinks. In this article, I will guide you through the different causes and what to do to get rid of the smell from your tank.

Causes Of Smelly Tanks

One characteristic smell from fish tanks is the smell of rotten eggs. It often comes from the substrate, filter or it tells you that there are some problems with the bacterial culture in the water. All these are signs that something is wrong or out of balance in the aquarium.

It could be a dead fish lying around and is starting to rot. It’s not that common that this is the cause though since other fish often start to eat on the dead fish and it will be fully consumed before it starts to smell. You will probably remove the fish too before that state.

You could be on vacation and have an un-observant person feeding your fish. Then it’s easy to miss a dead fish and the rotting could begin.

Another reason, similar to the dead fish problem, is dead plants. Plants are living organisms and they could, like every living organism, die. When a plant dies it will go to the bottom of the tank and the rotting process will begin.

As soon a dead plant falls to the bottom it will just lay there and slowly decay and will not be eaten by most of the fish. Snails and shrimp eat dead plants and are helping to remove the plant though.

Since a lot of plants are harder to digest and slower to eat for the animals in your aquarium and most have quite a lot of plants in their fish tank it will take longer for a dead plant to be removed.

A dirty filter is also a cause of a stinky tank! The filter has an intake where the water will be sucked in and filtered through a filter medium and then blown out into the fish tank.

What makes the filter smell is that leaves and dirt are getting sucked in with the water and are trapped in the filter medium and it is starting to decompose in there.

With a smelly tank, it could be a problem with the bacteria in the fish tank. Bacteria are eating waste in the fish tank. There are billions of bacteria in the aquarium. Most of them are in the filter, in the substrate and on the furnishing in the fish tank.

An imbalance in the flora or not enough bacteria makes it harder for the bacteria to consume the waste and the waste will build up and, you guessed it, starting to rot and make the tank smell like rotten eggs.

Bacteria could be the cause of a stinky tank too, not only the lack of them!

Having a bluish-green mat on the substrate and on the furniture could be cyanobacteria which are common in fish tanks. The cyanobacteria could give the tank a musty smell and indicate toxicity (source).

How To Fix A Smelly Tank

First of all, you have to check if it’s from the tank or from the surroundings around you. It might be the waste truck driving by or maybe a smell from your drain? Take a sniff in the fish tank and you will find out. If it’s from the fish tank it is a lot of times due to bad maintenance.

Always remove dead plants and fish as soon as you see them dead since they take longer to be digested from the inhabitants in the fish tank and is causing the smell. At least take some seconds every day to watch over your fish count and check if there are any problems with your fish, plants or furniture. You will prevent mishaps like a dead fish causing a problem.

If you are smelling a bad smell from your fish tank, the first thing you have to do is a water change of about 50%. You have to watch over your maintenance schedule. Is it a period where you have been bad with the maintenance? Fix it! make a day in the week you “water change day” where you spend a small time changing about 20% water. Your inhabitants in the tank will thank you for it!

Once a month during the cleaning you should clean your filter. Only clean it with the water from the fish tank! Never with water from your tap! If you clean your filter in tap water you kill a lot of the bacteria living there. The tap water has substances killing bacteria off making it safe for you to dring. If you clean your filter with tap water it will also kill off the good bacteria living in the filter.

The goal with the cleaning is to remove the waste that follows the water into the filter and gets stuck in the media. It’s not to remove the bacteria since it’s not the bacteria that smells, it’s the waste!

If you have medicated your fishes, the medicine will kill a lot of the bacteria. That’s what medicine is for! To combat that, you could put a filter starter in the filter to help the bacteria to grow back faster and prevent insufficient bacteria flora. You could put in a filter starter at the monthly cleaning of your filter to help the bacteria further but it’s not necessary for a healthy tank.

Another tip is to put activated carbon in the filter. What it does is that the carbon will absorb any contaminants in the water that may be the cause of a smelly fish tank. You need to change the carbon regularly, once every month for example, since the carbon will get saturated rather quickly.

As in the tip with a filter starter, it’s not necessary for a healthy tank. It will not damage anything though. Use it as a temporary solution to get rid of the stinky smell of your fish tank.

Are You Having A “Fish Soup” In Your Tank?

If you have too many fish in a tank you are making a “fish soup”. It could also be that there are too many different species in a single fish tank. It will not look good and will come with problems.

Since there are so many fishes in a limited space the waste will build up a lot quicker. It makes it hard for the bacteria to break down the waste and starting the nitrate cycle. It will build up ammonia in the water and might be lethal for the fishes.

How many fish to have in a tank differs depending on the size of the fish and what kind of fish you are having. But a rule of thumb is 1 inch fish for each gallon of water, or 1 cm fish for each liter of water.

Since the waste is building up there is a risk of a stinky smell coming from the fish tank. And since waste is building up there is an increased amount of food for snails and you might get a snail infestation.

A snail infestation is not bad but a lot of people are not liking the look of a lot of snail in their fish tank.

So, be careful with how many fishes, plants and how much you feed your fish. Also, be strict in your maintenance and do it regularly to don’t have the risk of getting a smelly tank. A smelly tank is not nice for you, others or the fishes.

Related Questions

How often should I clean my tank? One to two times a week by siphoning the gravel and change about 20% water

What fish help clean the tank? No fish eat waste but snails do. Fishes that are good at eating algae include Otocinclus and the Siamese algae eater.


I have been a fish keeper a big part of my life. During the years I have gathered and searched a lot of information about fish keeping and here, on the website, is where I share that information to help you with your fish keeping. I want to provide the information that I didn't get when I had a question about my fish tank.

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