Why the hospital tank looks so empty
Every missing feature of a hospital tank is missing on purpose. No gravel, because a bare floor shows you exactly what the fish is passing and lets you siphon waste in seconds. No plants or rough decor, because a weak fish needs nothing to injure itself on and you need an unobstructed view. Small size, because the fish is easy to observe, reaches food without travelling, and the frequent water changes this tank demands stay quick and cheap.
The one comfort you do provide is cover: a smooth flowerpot on its side, a clean pipe section or a soft artificial plant, so the patient can choose to be hidden, which sick fish strongly prefer.
The kit, and the water it holds
Fill the tank with water taken from the main aquarium, not fresh tap water, so temperature and chemistry match what the fish is used to and the move itself changes nothing except address. Add a heater set to the main tank's temperature, since a small volume of water cools fast in a room. Filtration should be gentle: a sponge filter run from an air pump gives mild biological filtration and oxygen without a current the patient must fight, and it will not strip out anything the vet has asked you to add to the water.
A lid matters more than it looks. Sick fish can still jump, and a covered tank also keeps the light low and the world outside at a distance.
Moving the patient in
Net the fish with the minimum chase possible, guiding rather than grabbing, and support larger fish in a container rather than lifting them in a net through the air. Settle the tank somewhere quiet and dim, away from household traffic and television glare, and let the fish find its hide before you start watching in earnest.
Feed tiny amounts and remove anything uneaten within minutes. In a small bare tank, one rotting meal can undo your water quality overnight, and water quality is half the treatment in any hospital setup.
Running it day to day
Change a modest share of the water daily or as the vet directs, always with conditioned water matched for temperature, and siphon the floor while you do it. Each day, take two minutes to record the essentials: did it eat, how is it breathing, how is it swimming, what do its body and fins look like compared with yesterday? Those four lines are the chart a vet actually wants to hear when you phone for an update.
When the course finishes and the fish is feeding, swimming and breathing like itself across several days, reacclimatise it gradually to the main tank's water before release, and keep watching it there for the first week.
Storing that daily chart in the fish's Pet Passport on KinsLe keeps the whole episode, setup to discharge, in one record you can hand to any vet who treats this fish next.