[papagimp]

More rock, and remove any/all biomedia from the one filter and the biowheel from the other. Let the rock and sand filer the tank, use those HOB's for mechanical filtering and clean them often, weekly at the minimum. Jamiep had a great idea too, use one of them, preferably the larger, and convert to hold some macro algae to help reduce nutrients.

fwiw, my 20g high, with a biowheel sits with high nitrates, about the same as yours. Fish only with few snails and they don't mind my nitrates like that. Starfish may not appreciate them much though but i've got no personal experience with that type of starfish.