To add to that, the reason your fish appeared happier in the 5 gallon is because they were in that one long enough for it to have cycled. I suspect this one has a reading for ammonia (test that). Your ammonia and nitrIte should both be 0. Anything more than that is extremely toxic and the cause of your fish' problems.
Though the tank size is not directly to blame for this AT THE MOMENT, your fish are already showing signs of ill health due to it. Indirectly, for example, they are stressed and water quality has deteriorated due to the inadequate size of the tank (a larger tank would be easier to keep stable). As a result of this, your fish now have septicemia. Also, you say you've had these fish for 6 months - yet they are only 3"? Goldfish grow extremely fast if kept in a large enough tank and can reach their full size within a year.
Besides doing water changes, stop the bacteria treatment - it's for internal bacteria and that's not the problem - and also gradualy reduce the water temp. back to what it was. That treatment has the side effect of also killing off some of the good bacteria that make up your biological filtration - this won't help the tank cycle any faster so water quality won't improve any sooner. The higher temp. reduces the oxygen content of the water - it also speeds up the rate at which bacteria multiply and enzymes work (which is both good - as the good bacteria multiply faster and convert ammonia to nitrIte to nitrAte faster - and bad - because food rots faster, ammonia is produced quicker and any bacterial infections take better hold). In this particular case, I think the cons out-weigh the pros for increasing temp. Besides, increasing temp. only has a significant effect on the treatment of ich.
Just to emphasize something someone said earlier (sorry, can't rember who right now) - when you do your water change, make sure you are using a water conditioner to de-chlorinate your tap water before you put the water in your tank. Also make sure that, if you rinse out your filter sponge/media, you do so in OLD TANK WATER - NOT in tap water which contains chlorine and thus kills off all the good bacteria that grow in your filter and are essential if you want your water to remain clean and safe for the fish.
Read the links in my signature about cycling if you don't know what that is. I also suggest you take some of the advice being given to you seriously because, regardless of what your lFS may have told you, I'm sure I speak for everyone here when I say that all we want is for your fish to survive and be healthy and happy.