I was reading an article by Ted Judy about raising apistogrammas, and it mentioned a "sponge filter." I have looked around at the local pet shops and found only one filter called a sponge filter. It is essentially a powerhead with a sponge attached to the bottom. I was wondering if when you refer to a "spong filter" this is what you are talking about. Also, I am planning on raising some apistos in a 20 gallon long tank and was wondering how many sponge filters I should use. Thank you very much for your time.