cyano?

[SpankMonkey]

does this look cyanobacteria? aside from perfect water chemistry, what would cause it to bloom. weekly 50% water changes. not overfeeding. dont want to nuke it with chemicals.

 

[blaxsun]

Yes. It's usually caused by overfeeding/overstocking. A UV would help, but since I don't know your tank size, stocking, water parameters or equipment/filtration this is just a general guess.