I know it's off the topic and I may get reprimanded but if someone doesn't like a thread or thinks a person is beneath another-then just don't reply!! Were supposed to be talking about fish-not insulting people's intelligence-por favor deja la gente en paz!
Sponge filter is just a sponge, round, with a stem in the top to attach a powerhead to pull water through it. The brand I know is Hydro and they're pretty cheap. Mostly for bio filtration.