What is this? Is it harmful? How do I get rid of it?

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Sep 20, 2021
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This white stuff showed up the other day, I think it was maybe the day after a water change? Does anyone know what it is or how to get rid of it?

I first noticed it on the plants (none of the plants are new) and now it seems to be on the substrate too.

My tank currently has 3 old guppies (I did have a larger number but they’ve been slowly dying of old age the past few months), two nitrite snails and three amano shrimp. The cleanup crew seem healthy and happy enough.

Nitrates were a big high the other week but it has been testing fine since then. I have a sponge filter, heater and light in the tank


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It looks like something precipitated out of the water.

Is it hard or soft?
Does it wipe off?
It does wipe off, it’s just very hard to do so with it being on the substrate and on the plants
Looks like a bacterial or fungal growth. It appears to be growing on the plants, substrate likely anything with a biofilm on it. It could be the result of high nitrates. If I am correct short of sterilizing the tank I don't think there will be a way to kill it off other than removing its food source, which would mean keeping your tank very clean with frequent and large water changes. It might be good to clean your filters as well. Don't do anything drastic that may affect your cycle.

Some better photos and better history leading to this issue might be helpful to others trying to help you with this problem. When you say your nitrates were high how high are we talking?
It look like some sort of early stage of water mold.

If it is, it's going to be a permanent guest... Some peoples have it in their tanks for years.


Overfeeding is the principal cause in many cases. It could also take advantage of wound or weak fishes.

I like to use an holistic approach in conjunction to mechanical removal. At some point they will make the difference between winning completely and a constant battle.

Introducing copepods like Tigriopus Californicus can help tilt the balance in your favor. While they are not going to feed on the bacterial or fungal growth directly they are still broad consumers of various forms of detritus, including decaying plant material, uneaten fish food, and other organic matter. They are small enough to go places snails and shrimps can only dream of. Are direct nutrient competition to your mold problem. And are food for guppies. Since they have short lives, their population regulates quickly.

Another good way to counter it, is increase water movement and oxygenation. Your Sponge filter might not create enough water circulation in the whole tank and mold spores can deposit and germinate. this increase the overall risk of infestation. The delivery of more oxygen can contribute to create a environment less conductive to fungal growth. And the stronger water flow removes more debris form the water layer.

You seem to have a pretty good mulm layer in your substrate (not enough light to see well) a vigorous vacuuming of it and larger water changes should remove a lot of it's nutrient source. Try to feed even less and have nearly no waste after feeding.

You can also rinse your moss and plants in tempered tap water to remove the current build-up before it starts to digest them.

In severe case hydrogen peroxide dips might be necessary.

Malachite green can also be used against it. (note that it is really a last resort and will also kill all invertebrates in the tank.)

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