while we're tossing around ideas, how about this - if you don't want to use
gravel or sand (for ease of cleaning perhaps), maybe throw in a few 'bio-
balls' or floating plants, to help with surface area for the bacteria? that
way when you change out or clean your sponges, it'll buffer the effects of
bacteria loss. another idea is to use two sponges, or one cut in half, that
you clean one at a time.
cheers
-andrew