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I filled the tank with water from my current aqaurium and put in an air stone. Do you need a filter in a hospital tank if your are doing frequent water changes?
You don't need a filter, but you do need beneficial bacteria in the quarantine tank. Just the water from the old tank won't help. The beneficial bacteria is built up in the filter of the established tank. You could buy a really cheap filter, and stuff some of your media from your current filter into the new filter. If you start fresh your goldfish will be harmed.