Black Slime Algae

[Jack]

Yeah, my local dive shop has black algae like you are talking about, they don't use RO/DI so I would guess that would be your problem after your break in period. Even if you use pure water, you still go through a bit of algae stages, it will go away. Also, algae grows in low/stagnant water, so maybe direct some powerheads or your return line near that to change things up. I think it will be fine and go away on its own though.