Black moor resting by heater

Hi! I'm relatively new to keeping an aquarium with fish, and here's my
question:

We have 2 black moor goldfish. One of them we had about a year in a 1-gallon
aquarium, and he did fine. A few weeks ago, we moved him to a 10-gallon
aquarium and added a second black moor. About a week ago, I noticed that the
original black moor seemed to be having trouble swimming against the current
of the filter. Upon closer inspection, I noticed that his tail seemed
frayed. I phoned the pet shop, and he was diagnosed with fin and tail rot
and I am now on day 3 of 5 of treating the aquarium with tetracycline
tablets. The sick fish seemed to be doing better as soon as I started
treatment but then I noticed he was in the corner by the heater, by the
surface of the water, not moving. I thought he was dead. But then, he began
to swim around, frisky as ever.

In the last 2 days, he seems to be spending an awful lot of time "resting"
in the corner by the heater, but then he periodically swims around and looks
as healthy and happy as ever. His tail seems to be regenerating (or am I
imagining that ?) and his eyes seem clearer (or am I imagining that too?). I
am inclined to believe if he were going to die, it would have happened by
now. He seems to be recuperating, so what is going on here? Is he staying by
the heater for warmth? Is he tired? I thought maybe he was having trouble
fighting the current being generated by the filter, but even when I turn off
the filter, he stays in the corner. The other fish is swimming around
normally.

Any thoughts on what is going on here?
Best regards,
---Cindy S.