A few things:
-Those two powerheads should be enough for you tank,
-Are they alternating or both constant on? Alternating is best.This results in a random flow which is best for keeping things suspended. Don't disregard placing them both on the same side of the tank, low/high..but alternating!.
-Also ,inspect your flow visually; if the sand bed has slight movement along with good movement of your coral, there is enough flow..so I would check your water parameters, as this could be the cause of your algae buildup, not the flow of the pumps.