Having used a WB stove about 10 years ago to heat for several winters, I know that the room temps can get pretty hot at times. I didn't have any fish at that time so don't have any personal experience. Are you using the stove strictly to heat the room or the house? Obviously if it is only to heat the room, you wouldn't ever have it really hot in there. I used mine to heat the entire house (about 1400 sq ft) so when I stoked it good at night before going to bed, I know the temp in that particular room probably got up into the upper 80s or 90s. Water holds temperature very well so short term high heat shouldn't raise the temp on a 55 gallon too much. I would definitely set the tank up as far from the stove as possible though just in case.