Using .5tsp/gallon is like taking half an aspirin when you need two. I doesnt do anything, and you build up a tolerance to it.
1tsp/gallon is fine, dont be worried. And to be honest, I wouldnt use less than 1tsp/gallon, as it doesnt do enough.
If you have plants, I would just remove them or remove the fish for the time hes being healed.
Ive never heard of it upsetting their digestive system. It will, however, cause liver and intestinal damage when used for long periods of time, much like any "medicine" would when done too long. Thats why 10 days is usually recommended.
he'll be ok!