i have no problems what so ever with nitrates in my nano, your problem is you have fish, true nanos have no fish! imo i would add a small skimmer to your fuge, make it a sump/fuge. by adding a small skimmer to my setup with a fuge already there, made the water crystal clear vs. kinda clear.

as for the fuge, all you need is 3/4" intake and 1/2" out