9. Solar heaters are a simple and low cost way of heating the fish water. Tilapia will eat 25% less is the water is in the lower 70?s, 50% less in the 60?s, stop eating in the 50?s and die below 50 degrees. If the fish stop eating, they don?t produce ammonia, which is the building block of the whole system. Water temp is also important for bacterial growth.