If thats the case,
Run one at 40gph per watt....
10-15 gallons of water per watt...
so either a 25(at around 1000gph) or 40 watt(at around 1600gph) will do fine. The 40 watt acting faster. Give or take a couple hundred gph wont hurt either.
I run a 25 watt at 1100 gph on 350gals of water for example.
Since it takes around 30000 µW/cm2 to kill algae, and the rates given are around 50000 µW/cm2, you'll have no trouble with either sizes.