I use two sponges per tank. Each tank has one sponge cleaned on an alternating schedule. This method always ensures that the biological filtration is never compromised.
I know you are on a budget but one of the best investments your can make is to get a linear air compressor, use one inch diameter PVC pipe and drill and tap holes for screw in air valves. Even a small unit will provide you with all the air you need now, allows for expansion and remember that if you are breeding fish the fry need brine shrimp and the hatchers need air.
Foster and Smith sell Oxyen Plus sponge filters with a discount on six or more.
They come in nice range of sizes for small to large tanks. I have found that
Model #9 is perfect for ten gallon tanks or less and Model #6 is good for 20 gallon tanks and up. The more you standardize the easier it is to run a fish room. Another Hint: buy half as many spare sponges as you have filters. This allows you to rotate the sponge through a use, clean, and dry on the shelf spares cycle. When I pull half the sponges for cleaning I grab a clean one off the shelf. I wash my sponges in the washing machine. Use only bleach, warm/warm, heavy duty cycle. Then a short rinse cycle with some Prime added to neutralize any traces of bleach. The come out like new.
Recommend the Stealth Heater because of their reliability, unbreakable and compact size. I use only 100 watt Heaters. They work on everything from 4.5 gal spawning tanks to 35 gallon tanks. I increase the wattage appropriately for tanks larger than 35 gallon.
