nitrates are driving me up the wall

[68shelby]

Yes I have a 45 gallon sump that will have a refugium in it as soon as I can get a light wired in. Right now the sump is basically 5-10lbs live rock rubble (inlet) under and over to the skimmer chamber then over to the refugium bed (currently about 4-5" of sand/gravel and then over into the return pump chamber.
And yes I have virtually 0 algae in the tank. even the back wall that I would leave for the tangs is bare other than pink and purple coraline. I wish I had some algae for the tangs.
Water flow is 2000 gph by the main pump,(probably somewhat reduced by the fact its in the basement) and I have 2 power heads one on each side of aquarium. 100g corner round.
Water I buy at sobeys. I have tested it prior to water changes and it tested fine. I am starting to think it is the mysis shrimp I have regularly fed.

[i have crabs]

how much rock in the tank its self?, i would invest in a ro/di unit so you can control your own water i woulnd trust buying water, not to mension the cost.
growing alge in a fuge will probably be enough to get the nitrates down from even the 20ppm mark but sound to me like there isnt enough filtering going on now