Is this diatoms?

[rogeragrimes]

2-year 120-gal. old fish/reef aquarium, with perfect chemical levels. I lost all my fish due to a power outage from Irma (future power issues now resolved). Eighty percent of my corals survived. I let my tank sit without fish for 6 months. During that time it really thrived and everything grew beautiful, and continues to do so now, with lots of fish. While it sat without fish, some brownish/yellow algae/diatom-looking stuff (?) started growing in one area of the tank. I know that most issues such as this are due to over feeding. At the time it starting growing, I was either not doing any feeding, or just "reef chili" where I would put in 1-2 tiny scoops of reef chili every other day...so I don't think I was overfeeding the tank. The unidentified stuff has only continued to grow and spread across my tank bottom. If I vacuum it up, it comes back into the same places. I notice that each morning it is nearly translucent or almost unnoticeable (because the lights are out)...so I know it's photosynthetic. During the day, it gets darker and darker and spreads slowly. What is it (see attached picture)?

[Michael Hoaster]

That's pretty dark. It may be dinoflagellates. Does it produce little oxygen bubbles later in the day? Does any form on the glass?

[bertoni]

I agree that dinoflagellates are a possibility. I doubt that it's entirely diatoms because they don't form films like that. Also, many snails (like Trochus) are happy to eat diatoms. I'd probably cut back on the feeding and work from there. Sometimes GFO helps, as well. If the corals are doing well enough, I'd avoid any drastic changes.