I have a 25 gallon fish tank with one 4" piranha in it and that it.my filter is for 30-60 gallons but it is kind of old. i just recently started getting some brown nasty stuff on the rocks and plants so i cleaned my tank really good. 3 days later its back and now its covering the whole tank and its nasty. its like a thin brown gunk and idk what to do! Please help me!!!!!!!
-Nate
-Nate