i have a 25 gallon tank with 3 albino buenos aires tetras, 3 spotted corry's, 3 black finned tetras and 1 blue apple snail. the problem i have and it is a big problem is everything turns black . i take everything out and clean it when i change water and i change about 1/3 every 2 weeks and after about a week it is black again. even the snail gets black. what can i do to stop this? it gets on everything, the glass, and all the things i have sitting in there and the stone. any help would be gret , thank you, Lloyd