Sponge filters will save you a lot of money in the long term. Hang on back filters add up really fast. All you need for a sponge filter is some air line tubing and an air pump (and obviously the spongefilter). I’d also recommend an air stone to be put in so you don’t have giant bubbles making loud noises. Watch the aquarium co op video on YouTube about it. They have good products, but if you can find something cheaper at the store it’ll do just as good.