Hospital Tank ??????

Ideally, it would be great if you had a hospital/quarantine tank running all the time, so that the tank is cycled for all 'visitors' & no ammonia problems would occur (however, when you add a lot of medications to a cycled tank, you can kill some of the good bacteria in the filter, which can lead to a mini cycle anyway). If you choose to keep the tank running all the time, you would still need to add fish food or something to feed the bacteria on the filter, or else they will die off. Some people use snails & their food to keep the tank cycled. Water changes would still be needed to keep nitrates down in the tank, and to remove unwanted medications.

Many people just use a QT temporarily for holding new arrivals or medicating, then tear it down after use. If you have a spare sponge filter, you can run it in a cycled tank for a while to get some good bacteria, then transfer it to the QTank when needed. Some people keep an extra sponge filter in one of their tanks at all times, so a cycled filter is available any time it is needed.

A warning--don't make the QT too pretty (ie. with nice decorations, etc) or you may end up keeping fish in it permanently (as I did), which would defeat the whole purpose!

So if I were to keep extra sponges in an established tank, and got a new fish, I could put sponge, new fish, maybe some gravel from established tank in with water and keep that as the q tank for the time Im quarantineing the new guy? What about if its one of the more sensitive types of fish?

If I kept the tank running with no snail or anything, how much fish food would i need to add? How often to do water changes?

Yes, you run a spare sponge filter in a cycled tank, and after a few weeks you can run it in the hospital/quarantine tank & the tank will be cycled for your new fish. The longer the sponge filter has been running in the cycled tank, the more beneficial bacteria will be on it.

I've never kept a tank running permanently without something in it, but I have used fish food to keep a filter cycled in my big tank while I was quarantining my fish in another tank for a few weeks. I used a small pinch every 2nd day.