There’s no need for a quarantine tank generally if it’s the first fish in because the new tank serves this purpose and if subsequent fish are coming from the same batch/shop then there generally shouldn’t be a problem. Catching, Transporting and New Introductions are general stressors for fish that can make them vulnerable to opportunistic infections that may already be present in the water (typically fungal or bacterial infections) that can become florid if fish are picking on each other due to hierarchy/food pressure or housing with a larger more aggressive/unsuitable tank mates.
As long as you keep your tank healthy and make sure that the fish you are choosing appear to be free of disease then you maximise your chances of not having problems going forward. If the fish do become unwell then generally treating the whole tank does the job but if it’s just one or two fish that don’t respond well to treatment then having a hospital tank can help.
I’ve never kept a dedicated hospital tank running but if I need one for targeting fish meds I can’t use generally in the tank (meds that may harm shrimp/snails) then any decent sized container suitable for the size of the fish and running a filter (preferably with cycled media) and heater if needed will usually do the job on a short term basis.
