I had to move my male veiltail to a different tank to treat his fin rot, but now he's darting up and down against the thermometer almost like he's scratching himself on it? Could this be a sign that he has something more than fin rot, or is it just stress from the move? His fins are not clamped, but they are fraying and black on the edges that haven't already fallen off

The tank is still cycling, so I'm sure that's what originally caused these problems.
The water at about 74 degrees (I can't really make it much warmer because it is a small tank, and my heater makes it too hot...), and unfortunately I don't have a kit to test the parameters with. However, I am doing frequent water changes in order to keep his water clean and prevent high ammonia and nitrite levels. I'll most likely be heading to the pet store this weekend to pick up some aquarium salt to treat the fin rot, but is there anything else I should look for to help him get better?
I just recently got him, and I feel awful for already letting him get sick! Please help!