Just to be clear, a hospital tank and a quarantine tank serve 2 very different purposes which shouldn't be done in the same tank.
A hospital tank is a tank where you need to medicate the fish for a disease or wound. It would be best to use in a quiet place in the home or wherever one keeps their fish so that the fish can be undisturbed as much as possible. It should be sterile when the fish goes into it. No substrate, no biological filter ( most meds kill the BB) and minimal, if any, decor. For the most part, the only thing going in a hospital tank is a heater and an airstone. Follow the directions on whatever medication you are using as to when you should be changing water. When you are finished with the medicating portion of the process, the fish should be moved to a holding/ QT tank until ready to be introduced or reintroduced to the main tank. At that point, the tank should be broken down and sterilized and left dry and unused.
A quarantine tank is just that, quarantining the animal to observe the animal(fish in this case) for any health issues and to allow it time to get adjusted to it's new surroundings, foods and feeding schedules, etc. (You can use any size tank for this.) This should be done in a biologically sound tank and for longer than 2 weeks. Some parasites have a 6 week life cycle so your fish should be QT for at least that long or longer ( recommended is 8-10 weeks if not medicating) to ensure you are not introducing anything into your main tank. Once the new fish have reached the end of this QT period, they can be introduced into the main tank and the QT tank can be maintained with fish such as cory catfish or other common scavengers that will keep the biological filter going and are not harmful to any other fish you may be putting into the tank in the future. Keep in mind that these fish can possibly be exposed to diseases and death so I would not use expensive fish for this purpose. They may need to be replaced.
So as you see, they are 2 very different tank setups.
As earlier suggested, using a 10 gal tank as a hospital tank would be a better choice for a number of reasons. The biggest one is that most medication's dosages are for 10 gallons of water. (That's 10 actual gallons of water not a 10 gal tank which doesn't hold 10 actual gallons of water.) Considering the costs of most medications, you don't want to waste any of it unnecessarily. Here's another hint, since most 10 gal tanks don;t really hold 10 gallons of water, you can do an actual measure for 5 gallons of water and just divide the medication dose in half and be right on the money ( assuming the fish you are medicating can fit in the 5 gallons of water.)
For pill forms, just cut in half. For powders, get yourself a cheap set of measuring spoons at the $1.00 store and measure out the dose then divide in half.
It saves money.
The second good reason to use a 10 gal is that they are small enough to just put away somewhere until they are needed again. Since you are only using 5 gals of water, you can set up the tank on anything that can handle about 50-60 pounds of weight. Airpumps are portable so the only thing you need to be close to is an electrical outlet.
So I hope this better explains these 2 setups and helps you make a wise decision. Make good use of the $1.00/ gallon sale.
