[hfp75]

-Any chance the sun is hitting your tank?
-Skim a darker skimate...
-Rowa is good, but how much are you using and how often do you change it? Keep your PO4 at 0.1 +\- 0.05, if your running lots of Rowa and your po4 isn't dropping just keep running it, maybe your rock or sand is saturated and your slowly unsaturating it - making the system healthier.
-Google : Red Slime Remover by UltraLife. It's good stuff but shut off your skimmer till it's done it's job. I end up running it once a year'ish.
-you got a sand bed? If so how healthy is it ? Ie: what's the sand turn over rate? (Sand star, sifting goby, snails, ect) Sand shouldn't just sit......
-Ever considered running a biopellet reactor? It'll eliminate your no3.
-snails in the tank? Turbo? Turbin? Ect that consume algae?
-what's your lighting?
-if your running ro/di I'd skip the carbon.
-Your skimmer doesn't really remove no3 it removes DOCs that will break down into no3, if there's no3 in the water the best way to eliminate it is either water changes or denitrifying bacteria.

Lots of stuff to consider...