Algae blooms in my experience whether it is FW/SW or the local pond can be traced back to excess phosphates. I don't see in this thread what your water source is, if it is DI or RO then it might be worth testing for phosphates, maybe a dirty or broken membrane?

i used to have about a dozen of these back when my phosphates and nitrites were thru the roof, before i got a skimmer and started zeo. for me phosphates were the main culprit, but as with all nuisance algae it really doesn't take much to get it rolling unfortunately:sad:

My source water is RO/DI. I test every jug before I use it, and it's running at 0 tds. I have a TLF PhosBan Reactor which has 2 cups of HC GFO that I change every 8 weeks. There are no detectable phosphates or nitrates in the water column. Not that that means anything... :lol: