Depending on stocking levels, and tank maintenance, you don't necessarily need a filter at all.
I use a variety of filters, depending on what is available, the size of the tank and what sort of fish are in the tank.
Do you have 18 tanks of goldfish? Are they separated by variety?
Corner and sponge filters are fine for smaller sized fish but larger fish need more filtration. I like HOB ('hang on the back' like AquaClear or Whisper brands) filters or external canister filters because I like to keep as much equipment as possible out of the tank itself.