I am trying to cycle a new 10-gallon tank with 3 danios. Every day for 3 weeks the ammonia, nitrite and nitrate levels have been 0. Now I have brown algae on the gravel. What could be wrong?
by 
Kathie Muench
ANSWERS: 2
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Your test kit for one. It takes longer than three weeks for the tank to fully cycle, you should have noticed an initial spike in ammonia and nitrite, followed by a slow drop in both levels down to zero. As for nitrates, those are the end product of the biological filtration cycle, and will steadily rise in your tank until you do a water change or put in a huge number of live plants. Brown algae growing on your gravel isn't a sign of a problem, it simply indicates that your tank is rich in nutrients (thanks to the waste your fish produce) and has enough light entering the tank to support photosynthesis. If you want to reduce algae you could always put in enough plants to "lock up" enough of the nutrients in the tank to prevent an algae bloom.
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Brown algea grows under low light conditions so either add a light or leave it switched on an hour or so longer.
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