In anything under 5gal you need to be doing 2 water changes a week to keep ammonia levels down. It sounds like poor water quality might be affecting him.
In a 1-5 gal tank one 50% and one 100% change should be sufficient.
How long have you been doing the daily changes and salt treatment?
A thermometer and a heater would also be a sound investment. You can get a thermometer for 2-3 dollars at a pet shop.
Otherwise I'm not sure what else to suggest, hopefully someone else can have some better advise.