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Its a little hard to tell from the photo if its algae or fungus, shot is a bit blurry, but above here is some hair algae I encourage my sump, I like it because it eats up nutrients like nitrate, and when it gets overgrown, I tear off and feed clumps to my fish.
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Many species of fish eat it (if that's what you have), mollies, American flag fish, many African cichlids. Many Africans will cultivate plots of their favorite kind.
One of the best ways to avoid it, are doing enough partial and frequent water changes to keep nitrates and other nutrients diluted and at low levels, also shortening the amount of hours the lights are on, unless you are keeping aquatic plants, the lights only need to be on when you want to watch your tank.
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