I have two of the stackable sponges... They're great. I have one in my apisto fry tank right now. The nice thing is that you can stack them in bigger tanks to get better filtration without taking up a ton of room.
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry