so do you have lights or light going into the filter.was wondering about possible algae.and as far as the bottom do you get any debree down there.its a sump right?if its not that accesable you would probably just want to leave it bare if it is low flow.maybe the ceramic media if it doesnt get any mulm down bellow.you could also put something as a prefilter from the pumps draw.some zeoite or carbon maybe.