Maggie, your temperature is on the low side, it ought to be 26 degrees celcius. Petsmart does sell the submersible tank heating pads, so that would work.
I'm not completely sure what the employee is saying is correct, but I'm not familiar with that particular med. Read the directions on the bottle and follow as that says, because with medications, if you aren't careful and dose incorrectly, the pathogens will become immune to the meds and come back stronger and harder to cure.
Because he is in a one gallon, you really need to keep up with your water changes. Complete the med treatment, and see if that works. If not, I highly
suggest you go to the natural method. We know what we're doing here. :)