confused

[asherah]

my tank is 44 gallons. It has a modified whisper filter turned into a refugium with cheato.
I usually change the water 1 every 2 weeks. More if I think it needs it. Usually about 8 gallons or so.
It's also kept at 78 degrees F.
They are fed : spirulina, nutrafin max marine complete food. When the angelfish was in there they were also fed nori once in awhile.

So what do I do to get the nitrates down ? More water changes?
Maybe actually having sand instead of the gravel will help ?

asherah