Flow is also important for heaters; if the water is not moving quickly, the warm water will not be distributed throughout the tank, the heater will think the water is warm, and will then shut off. I would also definitely add another heater. In my 125, I had a 250W on both sides of the tank and a thermometer in the middle connected to my chiller. The heaters are really only for night as the chiller operates during the day.