Most people just keep a spare 2ft tank as a hospital/quarantine. It may be left without substrate, but often at least a thin substrate will help keep fish more secure. An inexpensive filter setup is often the choice, I just use a simple sponge filter with a powerhead, but even an airstone will suffice. A heater is essential, as with many diseases a slight raise in temperature is part of the treatment. That is basically it. You can use a fishless cycle to start it, and seed your filter bacteria from your current tank. When the hospital tank is not in use, most people keep a few guppies or swordtails(any inexpensive fish) in the tank to keep it in cycle.