Ok i have 4 tanks set up, 2-10g 1-20g and 1-55g. I have Pea Gravel from wal mart home and garden Department in 3 of them. 2 of them are fine(10g and 20g) but the pea gravel has turned coal black in my 55g tank, the water source is the same for all the tanks, and the flow rate in the 2 that are fine is LESS than it is in my 55g. I cant understand why the gravel is turning black in one and not in the other 2. It came from the same bag, and is not "Aquarium Gravel" but it did pass the Muric acid test(no reaction)it also Smells pretty bad when mixed up. Depth of the gravel is similar in all three tanks. Any ideas why the tank with the highest flow is getting this problem and any ideas what the cause is?