I started a 5 gallon tank about a month ago for a betta and put a couple of these type of plants in there and a couple others. About two weeks ago it started getting this dark stuff on it which I thought was algae. Anyways, I ended up returning that tank because of problems and bought a 10 gallon with flourescent lighting and set it up a week ago. I washed all the plants off because the stuff washed right off and I planted them in there. After about a week I noticed this stuff coming back on them again.
I also have these plants in there:
and in the first tank they started to get the dark stuff on them too but haven't yet so far. I am not sure what these plants are. I bought them at Petsmart and they came in a tube. Anyways, what is this stuff and how do I get rid of it?