when it is grungy looking. I got a new sponge in my 75 soaking up the BB while I am in the process of setting up 20L betta/platy tank. So as title states, i mean it cant take as long as a filter in an un-cylced tank. I assume that it will cycle before it gets discolored by gunk. also a question to ponder, since i am using a cheap air pump; it is rated for 5 gallons and well the sponge is quite big. Will it take longer to cycle the sponge because less water is pulling through? should i get a air pump rated higher to get more suction so it filters more BB to cling.