Hospital tank questions

If you can fit in a 10 that would be great just in case you have a big new fish you want to add in the future.
Usually just a filter, heater and a thermometer are all you need.
No gravel and plant for easy clean up.

There's some info in the article here that may be useful.

Yeah, I would go with a 10 with no gravel or live plants. But have something in there for the fish to hide in if they feel the need. Fake plants are fine.

No gravel....just a nylon bag with some gravel in it for a place for beneficial bacteria to take up residence. It's easier to keep clean.

No live plants because if you have to treat a fish for something, some meds will kill your plants.

Definitely a heater and a filter that you can remove carbon from. Any filter that is rated for a 10-20 gallon tank should be ok.

I use a simple 10 gallon that I bought for $10 and a simple airpump driven box filter for $5. I keep a sponge in my main tank filter which I cut to fit the box filter. If I need to do some treatment or QT work, I just set up the filter with the colonized sponge and have an instantly cycled tank. As soon as I am finished, I boil the sponge for 15-20 minutes to sterilize it and return it to the main tank filter for recolonization. I sterilize the rest of the tank/equipment in a bleach solution for 24 hours. I rinse it very well and let the tank air dry until needed again. As a simple cheap cover I use eggcrates though a real glass canopy for it is not that expensive.

I'm keeping a spare mini penguin on my main tank with just the bio-wheel (no filter cartridge) to keep a bacteria colony. Should I need a hospital, I can transfer it over. I actually don't have a hospital tank at the moment, but should I need one, I can get a 10 gallon at a myriad of pet stores around for $11. In the meantime, I'll be keeping my eye opened for a used one.

Just saying that if money is tight, you don't have to get everything until the need is there (unless the remaining components are not quickly available to you.), but you should be thinking about how you're going to transfer nitrifying bacteria over to your hospital.

I agree 100%. You can pick up everything you need in five minutes at
wal-mart EXCEPT that cycled sponge. Get one of those air-driven sponge filters, and jam the sponge into your existing filter. It will be cycled whenever you need it. (It's a great way to start up a new tank without having to wait a month to cycle it too.)