I would put most of the blame on the use of tap water aswell as the high bio-load with small water changes. You're kinda stuck because you need to do larger water changes but you shoulnd't really use tap water.

Cheap RO/DI units can be found on E-bay from a guy in N. Vancouver under the name Aqua-Safe. I think there are quite a few members on this board that have one of these units and I'm sure they would reccomend them.

I would also add a bit more flow. A protien skimmer would help out too.