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I've had my baby RES for about 5 months now and he has a few basking rocks, but for awhile now they've started growing a dark green substance and I don't know what it is. It comes off with a light wipe of a rag/scrubber, and the only other things I have in the tank are smooth glass rocks covering the bottom, a pump and a fake plant. The pump also gets the green on it where the water consistently runs over it. Anyone else have this problem/know what this stuff is?

It's algae. Common where the light hits the water.

I thought so. Is it harmful to (my res) Gilbert? Do I need to clean it out frequently?

Shouldn't be harmful. I mean, no one cleans lakes.

True lol. Thank you.

You don't have too, and it looks nice sometimes. I wouldn't let it grow on the tank though, especially on silicone.