I have a 2.5 gallon I treat and quarantine corals in
I would recommend you get a system to properly QT fish from here on out.
Petco has great deals if you’re state side. They have 20 long tanks on sale all the time. If you order online and pickup in store you can get an extra 10% off. With that said a 20long is the perfect tank if you have room to set it up.
For HOB (hang on back) Filter I use the Fluval AC50. It comes with a few things to it. I only use the bio sponge that comes with it. The rest is media and other filtration stuff that is not needed for a QT tank.
For more clear water and to help reduce algae bloom or bacterial blooms once the tank is set up. I use Aqua Top Intank UV. 7watt version works for the 20 long tank. The super nice extra benefit is it create great surface agitation for gas exchange to take place and get plenty of o2 in the tank water.
For heaters I have been using Ti heater for about 1.5 years now. Prior I have done the glass versions. But no longer. The most accurate heater I have to date is a IM Hilos controller and Finnex Ti heater. But there way cheaper system that can work. The Finnex with the built in controller is a bit cheaper and has a build in fail said for over heating from what I recall.
Light, as long as you can get something with bright white lights you’re good to go. And intend ti limit lights to 6 hours for fish.
Bacteria I like to use turbo start 900. Give is a few days before turning on the UV. Like the Aqua Top in tank UV because it has a UV light turnoff separate from the filter pump. So it can be used to again creat surface agitation. Since you’re at an emergency stage…. I would do water changes ever day after the first two three days. This will keep ammonia down while beneficial bacteria catches on to the HOB sponge and other surfaces it can. I actually started to do Genesis rock in my QT systems for. It does not absorb any copper.
Get
copper power or copper safe.
For the fish some large 2-3” PVC fittings. Gives them hiding spots and less stress on the fish.