im guessing at the carbon is saturted and redepositing junk in the tank... imo pull the carbon, and give all the media a good rinseing... replace the carbon with biomedia. vaccum your gravel everytime you do a water change, and do alot of them until your perameters get back in control... nitrites that high suggest you had a prior ammonia swing... ammonia spikes can bomb your bacteria and restart a cycle... imo this is whats going on. your back to square one with a heavily stocked tank. stop feeding and do massive water changes the next week or more and keep testing.