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I'm a big believer in QT'ing fish, and indeed anything "wet"
R2R has a sub-forum for fish disease, treatment, and QT, as well as a Fish Medic team to help:
https://www.reef2reef.com/forums/fish-disease-treatment-and-diagnosis.771/
A QT tank need not be expensive or elaborate. For your sized display tank, I think a 10 gallon QT tank would suffice. You can get a 10 gallon standard tank from PetCo when they do their half off sale for about 12 bucks American. You'd need at least a decent heater, ideally on a controller to prevent a malfunctioning heater from over heating the tank, but I just use inexpensive haters in my QT tanks. I like the Marina brand heaters for this; inexpensive, and work well for me without an additional controller.
Some rely on water changes to mitigate ammonia, but I'd rather have a mature bio filter on my tanks. I like to use Hang On Back filters, specifically the Hagan/Fluval AquaClears. I like to keep some plastic bio media in my display tank(s) in chase I ever have to quickly setup a QT tank. I like to use plastic kitchen pot scrubbies from my local doallar store as bio media, keep a couple in your main tank for a couple months, and they'll be well matured to run your QT tank. You can safely reused plastic bio media after dosing medication like copper, but I just pitch the kitchen scrubbies once the QT is over.
A fish QT tank does not need to be lit, and indeed no light may reduce stress on the fish while in QT, with the exception that it's helpful to be able to turn on a light to observe the fish for signs of illness. A simple, basic freshwater light strip is fine for this, no need for expensive reef-capable lighting here
You'll need to offer you fish some hiding spaces to feel secure. Again, plastic is favored here, with PVC plumbing fittings often used; they won't react with medication often used to QT fish.
This is the R2R fish QT protocol with more information on how to setup a QT tank. I prefer this paradigm to others because if you ned help, the author(s) of this thread are right here on R2R if you need them. Certainly, please post back if you have further questions
#fishmedi
#fishmedicck!