Many people like the idea of a quarantine tank, but many people think it is too expensive or they dont want to buy a whole another tank just for temporaily housing fish. They are actually very wrong
For a quarantine tank a 10-20 gallon tank works for smaller fish (danios, tetras, rasboras, some dwarf chichlids) while a bigger 29-40 gallon tank are for the bigger fish.
All you need to buy for a quarantine tank is a very cheap filter, a spounge filter will be best, you can use an air powered or a powerhead spounge filter. These normally go at around 20 dollars. Another thing to get is a very cheap quality HOB heater, since you wont be using the heater much it has a low chance of breaking down or something. That is all you need to buy, the price goes at around 40 dollars. Also if your main tank has two filter you can simply move one of them to the quarantine provided that the other is capable of keeping your main tank clean. The last thing to buy is a tank, you do not need a cover, just the tank itself with nothing else.
You wont be needing a stand, just place it on the floor somewhere that you can.
When you buy all this stuff, store the tank somewhere else, keep the heater with the tank and place the spounge filter in your main tank. Whenever you need to use the quarantine tank all you need to do is move the spounge filter from the main tank to the quarantine tank (the sponge will be full of beneficial bacteria so you probably wont need to cycle the quarantine tank) and install the heater, add water. Once this is all done you may add the fish. Add some decorations from your main tank to the quarantine, add around 2-3 handfuls of gravel to the quarantine tank from the main tank. The decor and gravel will have beneficial bacteria on it which will mean the quarantine should be fully cycled in minutes. Now just get some cardboard or something and make your own cover. Get a desk light and place it on the side, but a light isn't really nessicary since you wont have any live plants in there and the room light should be suffice.
Congratulations, you now have a quarantine tank, when your done with it, wash and scrun all the decor that was in it, wash the sponge, boiling the stuff wouldnt be bad and let it all dry up. Once that is done you can place it back in the main tank and let bacteria build up on it for the next use of the quarantine tank. You can turn off the heater, dump the water out of the quarantine and store it for later.
I did this and all i spent was 10 dollars for a 10 gallon quarantine tank.