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How To Treat White Cotton Like Stuff In A Fish Tank!

A large number of people have recently got into fish keeping with many of them experiencing the problems that more experienced fish keepers overlook or treat without a second thought for the very first time.

This has resulted in a large number of beginner questions being asked about fish keeping and one of the more common ones that we have noticed people asking is about the white cotton like stuff in a fish tank that seems to grow at a rapid pace.

Although many people use the blanket term of “cotton wool disease” to describe these algae, funguses, mold, and bacteria growths, they are actually all different things that need different treatments.

The term cotton wool disease is specific to columnaris bacteria that causes the white cotton wool growth on your fish.

The most common thing that causes the white cotton wool balls in your tank is simply mold growing on any excess food that has fallen to the bottom of the tank and been left there.

Thankfully, this is very easy to treat and usually not a serious issue to your fish unlike actual cotton wool disease that can prove fatal.

What Is The White Fuzzy Stuff In My Fish Tank?

The white cotton wool like growths on the bottom of your tank is usually just mold growing on excess food that has fallen to the bottom of your tank. It is commonly a sign that you are either overfeeding your fish or not conducting tank maintenance regularly enough.

Thankfully, this mold usually only focuses on eating the fish food that has been left at the bottom of your aquarium for a couple of days and has no interest in your fish or your live plants in the tank.

Although it is unsightly, it is not as much of a problem as many beginners to fish keeping initially think.

Even aquarium water with perfect conditions will have a high chance of having these tiny mold spores in it as they are very difficult to get rid of completely.

The only reason that more experienced fish keepers don’t have issues with the white cotton mold growing in their tanks is due to having perfected how much food to give their fish as well as sticking to a strict tank maintenance schedule to prevent the discarded food being at the bottom of their tank for long enough for the mold to grow.

What’s The Best Treatment For White Cotton Like Stuff In My Fish Tank?

There are two options for treating white cotton mold in your fish tank with the first being to feed your fish less.

The mold is growing due to excess food sitting at the bottom of the tank long enough for the mold to grow. The second is to clean your fish tank once per week to prevent the build up of discarded fish food being there long enough for the white cotton mold to grow.

There is a third option that may work well depending on the other fish you have in your tank and that is to add a bottom feeder to your tank.

New additions to your tank such as a pleco, cherry shrimp or nerite snail will rapidly eat the discarded food on the bottom of the tank well before the white mold can grow.

Adding a bottom feeder to your tank is not always a suitable alternative though and it can be difficult to work out how to feed your fish to prevent overfeeding without underfeeding them.

This is why the easiest way to prevent white cotton mold from growing in your tank is to use a cheap gravel vaccum once a week to clean out all of the waste sitting on your substrate.

Why Is There White Stuff On My Fish?

If you have white cotton looking growths on your fish then it is highly likely to be due to the columnaris bacteria that is commonly known as cotton wool disease.

The bacteria is surprisingly common and usually added to your tank via new fish, plants, accessories or substrate with it quickly taking hold of the fish.

Thankfully, although it looks unsightly, in healthy fish cotton wool disease is usually very easy to treat.

We would highly recommend that you use API Primafix as your treatment of choice for cotton wool disease as it is very cheap, works within three to five days, and has an excellent reputation amongst fish keepers all over the world.

The dosing instructions on the label of the product are also very easy to follow based on the water quantity of your tank with the lid of the product acting as a single measure of the treatment.

You then multiple the treatment measure by the volume of your tank and you should quickly notice the white cotton growths disappear on your fish within a week.

Why Is There White Stuff On My Aquarium Plants?

The most common white cotton looking stuff on aquarium plants is usually just white string algae build-ups that are very easy to remove.

There is a chance that discarded fish food could have landed on your plants too and that white cotton mold is growing on the discarded food sitting on your plants too.

We have a dedicated article on dealing with white string algae in your fish tank but the quickest, easiest, and cheapest way to treat it is to use a product like API Algaefix.

The product will be able to rid your tank of the string algae in as little as a few days for smaller tanks but may take up to a week for larger tanks with dosing being very easy if you follow the instructions on the label.

If you take a close look at your aquarium plants and notice that there is discarded fish food sitting on them then it is highly likely that the white cotton stuff is just white cotton mold.

As we covered earlier in the article, this is due to you either overfeeding your fish or not cleaning your tank frequently enough and the treatments covered earlier in the article will be able to get the job done.


That brings our article going over dealing with white cotton like stuff in your fish tank to an end. We have covered the quickest and easiest methods to treat the three main types of growth that look like white cotton balls with all three treatments being beginner friendly. Due to it usually being beginners reaching out and asking about how you treat these cotton like growths, you should easily be able to identify the cause of the problem in your tank as well as how you are also able to treat the problem quickly.