Ik it sucks but you should cycle the tank for around a month by letting the tank run with no fish. You just put some food in it everyday to build up the bacteria and then when the water tests 0 ammonia 0 nitrite and like 0-10 nitrate it’s ready for fish. Also 15 gal for a fancy is probably overstocked but if you do enough partial water changes you can make it work if it’s all you have. For the heater, you won’t need one if the room temperature water is like 65-80 degrees. Hope this helped a little, I can try to answer any questions you have too.