Algae needs excess nutrients
to feed off of. Mostly it's levels of po4
above .03 ppm
that causes algae blooms. No3
plays a small role also but IME only if it's above 20 ppm
Even though you may test po4
in the tank at 0 ppm
the algae may be consuming most of it to give you a false reading.
Have you tested your source water for no3
? (before and after mixing your salt)
I agree with Kurt that you are heavily overstocked for a 55 gal
even if all your fish are juveniles. Even if you are maintaining no3
levels with constant pwc
the amount of food needed to feed that many fish is going to contribute greatly to excess nutrients that feeds the algae.
The Protien Skimmer can't come soon enough. You need a powerful skimmer to help reduce doc
, clean your canister weekly, and reduce your fish load to 12" of adult length or you will be fighting algae forever.