New Southdown Sandbed

[FishHead1]

Two weeks ago I swaped out my substrate and added a 4" SouthDown sandbed. It is now growing an ugly brown film on the surface.
How long before this stops?
Tank is 180 galloons. Been set up for almost two years.
Nitrates are between 15 and 25
Calcium is 400
Alk is about 3.5
No PO4
No ammonia or nitrite

 

[fishpoo]

i went thru the same algae changes. Probably a day or two. I did a water change and carefully sucked most of it out, but i hear it will disapear by itself. It's just a diatom bloom. unless it's dinoflaggellates. no, i doubt it.