My guys are all in 3 gallon tanks. I use both Marina and Top Fin preset 8W heaters (depending upon what I could find at Petsmart at the time). The drawback is that they are all "preset" heaters, which means I can't adjust the temperature. They maintain the temp at 79F consistently though, which is fine for everyday use. BUT if I ever need to raise or lower the temp (to treat a disease), I'll have to buy a new adjustable one.
If you want to just get one heater, and not worry about having to pick up an adjustable one if your fish gets sick, you can get a 25W adjustable heater.
You'll also need a floating glass thermometer, like this one: http://www.petsmart.com/product/inde...ductId=2752255
As for water conditioner, Callistra specifically recommended Prime because it neutralizes ammonia temporarily. It will only do this for about 48 hours though. So you still need to do frequent water changes, in order to keep the ammonia level down.
If your fish is having trouble getting to the surface to breathe, you can lower the water level so he doesn't have to swim so far upwards. If you have a heater, you need
to make sure it's covered to the correct depth.
(If you decide to put him into a heated tank, he'll need to get acclimated to it. Do not raise the temp faster than 1 degree per hour, and limit the increase to no more than 5 degrees total per day. )