Why calm is part of the medicine
An angelfish is a tall, slow, deliberate fish that reads its tank constantly, and stress works directly against recovery: a stressed fish eats less, breathes harder and heals slower. During a vet-guided course you cannot remove the disturbance of treatment itself, so the goal is to make everything else in its world predictable. The fish that knows when the light comes on, when food arrives and that nothing chases it has reserves left for getting better.
Start by deciding where treatment happens. If the vet's plan allows treating in the home tank, the angelfish avoids the net and the strange bare box entirely, which is often the biggest stress saving of all.
Light, cover and the shape of the day
Dim the lighting or shorten the lit hours for the course's duration. Angelfish rest in the shade of tall plants in the wild, and a brightly lit tank with nowhere to retreat keeps a sick fish feeling exposed. Add or leave tall, soft cover it can drift behind, and resist tidying the layout; a familiar arrangement is a map the fish already knows.
Keep the room around the tank quieter too. Tapping, sudden movement past the glass and children peering in at a fish that cannot retreat all register. A background on the back glass and a calmer corner buy real peace.
Water steadiness above water perfection
Chasing perfect numbers with constant adjustments creates swings, and swings are stress. Hold temperature at its normal setting, keep up gentle filtration and aeration, and make any water changes the treatment requires small, conditioned and temperature-matched, poured in slowly. Test on a schedule rather than constantly, so your hands and test kits are not in and out of the tank all day.
Between steps, leave the water alone. The angelfish's body is already managing the treatment; a stable environment is the contribution only you can make.
Feeding and tankmates during the course
Offer small amounts of the foods this fish reliably takes, and watch an entire feed. A convalescing angelfish is easily outcompeted, and a bolder tankmate stealing every mouthful is a stressor you can fix by feeding at both ends of the tank or separating the bully for the course, rather than moving the patient. Remove uneaten food promptly so it does not foul the water the fish is breathing.
Skip all non-essentials: no new fish, no rescape, no gravel-vacuum deep cleans beyond what hygiene demands. If the fish stops eating altogether, breathes rapidly despite good water, or deteriorates visibly partway through, contact the vet before the next scheduled step rather than pressing on.
Keeping a dated note of each treatment, feed and behaviour change in the angelfish's Pet Passport on KinsLe means the vet can adjust the course from facts, not impressions, at every check-in.