how deep is your sandbed? red slime is not algae , its bacteria, it usually feeds off phosphates leaching from your sandbed, ..........
a phosphate remover wont work because it consumes the phos b4 it can get into the water column for removal..................boyds chemi clean is the only thing that worked for me, i had it so bad when it all died it turned my water brown , corals discolored from nutrient overload and zoo explosion, but everyone is fine now..................
with each month i am geting less and less, i use boyds the night b4 i do a water change everymonth, and i started vacuuming out the top layer of my sandbed every month with water change, i have tons of flow in a 120 ( 2 6060 tunze and 2 1200 maxijets) i also cut back on feeding once a day to once every 3 days, fish are all still fat
this stuff is nasty and seemed unphased by flow..............
ill never go more than 1 or 2 " on my sand again..........