It's good to hear the little fella is feeling better
He most likely perked up from being put in a new tank with new water, but if its also an uncycled tank, you will need to test your ammonia and nitrite levels daily to begin with, and change out however much water is necessary to keep them below toxic levels (well any amount of them is toxic, and 0.0ppm is what you want once the tank is cycled, but for now, under 0.25ppm is what you want)
If your bristlenose catfish have been raised in cold water, I wouldn't get a heater. They should be fine at room temperature.
What sort of gravel did you get? I found with my comet and a pleco in a 50+ gallon tank I had to remove all the gravel and have a bare bottomed tank so I could siphon all the poop out every couple of days. It used to build up in the gravel, and water changes using a gravel vac every couple of days couldn't keep up with their waste production.
Sand is fine for goldfish- they can't choke on it, and the poop sits on top for easy removal.
The best thing you can do for your goldfish is buy a master test kit, and increase your water changes. This will save you from continual battles with poisoning and burns from a build up of toxins, as well as diseases like fin rot and bloat etc.
Try and feed your fish some veggies every day- thawed peas squeezed out of their shells are a favourite
bristlenose enjoy them too. Make sure you give your bristlenose algae wafers in their diet too.
All the best with your adventure