I constantly get a high nitrate reading. In the last 4 weeks we have taken the following actions:

Increased the water movement by adding an additional pump
Changed the sand in the sump tank
Increased the water changes
changed the filtration floss to another type
Tested the RO water that we use and it is reading fine
Reduced fish feed

Over the past few months we have lost corals and have a dark red algae that needs to be cleaned out of the tank almost daily. In the past 4 weeks we have not lost any more corals however the algae problem is still there.

Does anyone have any suggestions as to what else we can do to get rid of this problem please ?