The deciding question: where does the problem live?
The choice between treating a whole tank and isolating one fish comes down to where the trouble actually resides. If the cause is in the shared water, shared substrate, or a life cycle that moves between fish, treating one fish and returning it to an untreated tank just re-exposes it. If the problem is confined to one animal, an injury, a bullying wound, one fish wasting while tankmates thrive, then the shared water is innocent and the hospital tank is the kinder, cheaper answer.
Signs the whole tank needs treatment
Treat the display tank when several fish develop the same signs within a short window, because shared symptoms point to a shared cause. Parasites such as white spot are the classic case: much of their life cycle happens off the fish, in the gravel and water, so medicating only the spotted fish leaves the next generation waiting for it. A newly added fish that has already spent days in the tank before symptoms appeared has shared everything it brought with every tankmate, which also argues for treating the group. Bacterial problems that follow a water-quality crash belong in this group too, once the water itself is corrected, since every fish's gills and skin met the same conditions.
When one fish should come out instead
Isolate the individual when the problem is plainly personal. A torn fin from a dispute, a fish being harassed at feeding, or one animal that is thin and fading while the rest look robust all call for a separate hospital tank where it can be observed, fed without competition, and treated without exposing healthy fish. Isolation is also the right call when the treatment itself would damage the main tank: many medications harm shrimp, snails, delicate plants, or the filter bacteria your whole system depends on, and some cannot be used with scaleless fish at all. A simple spare tank with a heater, gentle filter, and hiding place does this job well.
What whole-tank treatment asks of you
Treating a display tank is a commitment. Remove activated carbon, which strips medication from the water, and expect some products to stain silicone or dye the water for days. Watch oxygen levels, since both illness and treatment can lower them, and be ready with extra aeration. Test ammonia during the course, because some treatments set back the filter bacteria, and a healing tank that slips into a mini-cycle trades one problem for another. If fish deteriorate sharply mid-treatment, or deaths continue after the full course, stop guessing and contact an aquatic vet with your notes on what was used and when.
Keep the record straight for next time
Months later, the most valuable thing you own is an accurate memory of what was treated, where, and what happened, because the next outbreak will rhyme with this one. Note the dates, the fish involved, whether the whole tank or a hospital tank was used, and how the group responded. The Pet Passport on KinsLe keeps that treatment history in one packet with each fish's records, so a vet advising you later sees the full story rather than a keeper trying to reconstruct it.