If you are switching from a 15 gallon tank, I am assuming you have a baby bp (Less than 200g?). I would start with a 6qt tub or no bigger than a 16qt tub. It may appear small compared to the tank but babies, more than adults, need to feel secure and moving them can stress them out easily. Secondly, you will find it is a whole lot easier and lighter to clean a 16qt tub than a 41qt (although not as heavy or annoying to clean than a tank). The overhead lighting must have a thermostat as well.
If your room temp is around 77-80F, skip the overhead light and use the UTH with a thermostat. If your room temp is cold, you can use a 2nd UTH or I would start with a 5watts bulb or smaller. Play with it using a temp gun or digital thermometer to get the temps you need for the right wattage. You may also utilize a stand that would move the bulb farther or closer to the tub to achieve the temps. Lastly, there is a thread somewhere that teaches you how to set up overhead lights and tubs. You must use the lid to keep the tub locked and secured.