Hospital tank do I need a filter?

[bettasngoldfish]

Yes, an uncycled filter would offer no advantage, but if a cycled filter is available, it is certainly worth using.

Yes, I agree

I keep extra sponge filters running in some of my aquariums just for the purpose of quarantine if needed (that way the are already matured) I also have several different size aquariums and a couple stock tanks in case I need them.

But I also have used a tank without a filter if I need to pull a fish or have brought a new one in that needs medication immediately. It can be done with some extra attention to water quality which obviously means more work as it means more water changes. But it can be done.