If the fish is sick, I will not release it to the DT, just keep it in the betta box or move it to QT.
Why?
1. If you need to remove the fish to the QT or bath the fish, it can be easily done in a betta box. Less stressful. Caught the fish in the tank with full of rocks will definitely much more stress for the fish and exhusat the fish during catch.
2. Less contract with other fishes. But does not means that the disease may not spread to other fish in the tank.
3. Less stress from attack from other fishes in the DT. Most of the other fishes like to attack the fish especially when it is sick and new.
Normally, you need to bath it with yellow powder for bacterial disease in a separate container. How long? Normally depends on the fish condition and experience. Too long will sometimes stress the fish. If the fish is strong, you can bah it slightly longer.