In article , "Stuart Mueller"
writes:

I have a 15w and an 18w bulb on for about 13hrs a day. No CO2

That could be your problem. I've found that 10 to 11hrs works for my non-co2
tank. I also had the black brush algae until I cut back from 12hrs. I still
get very good plant growth.

You may also want to cut back on the liquid fertilizer. The best way to find
the right balance in your aquarium is to experiment. Everyone's experience is
a bit different.

Cris