That doesn't sound like feeding to much to me, more like not enough to me. You're getting around 500 gph from that Mag 5. That's a lot for that size nano. You might want to go down to a maxijet.

You don't have a DSB in your tank? You are relying on the fuge for all denitrification? If so, that's probably not enough capacity for denitrification. There's only one way to find out.