Black algae

I have a planted 20 gal tank using 2 flourescent lamps and CO2 generatpr. Have replanted for the second time now and black algae again has started to show on the leaves of the plants, just black spots not threads or beards.
Have cut lighting to 7 hrs and water change 5 gals every four weeks.
The fish load is six neons three zebras four white clouds and three cory cats a little heavy but fish all seem to be doing well. I really don't want to go to plastic plants any suggestions?
Nels

 

Need full list of tests/params.

amo
nitrites
nitrates
ph
phosphates

also what besides the c02 are you dosing with?

Quite often your issue will arise if not enough fertilizers are in the water coloum. The plant can't use the c02 since they don't have enough of the other stuff and then the algae makes use of it instead.

Make sense ?