Something to consider is this. The wattages that the Under Tank Heaters operate at vary by size and brand, so some will heat up faster than others and some will be able to get hotter than others. So, depending on which heat source you have the probe attached to, some may be getting too hot or not hot enough. That is why I always reccomend using the same heat sources when running multiples on one thermostat. If you want to use a variety of heat sources on one thermostat, then I would suggest setting it all up, and monitoring the temps for a few days to see just how they are going to do before putting any snakes in...