I've had my tank for a few months now, 10 gal with 7 male guppies. One of them died, I had eight, i accidentally caught him with the gravel cleaner. Anyway, I had 'new tank' brown algae for a while, and thanks to some advice from people here, I let it burn itself out. But now I have green algae, and spots of black on a few of the gravel pieces. The green isn't wispy or anything, and theres a lot more green than black. Is this a problem? Do I need to do anything about it?