In the world of physics in order to answer the question you would have to know the center of gravity of the trailer vs. the center of the axle(s) vs. the center of gravity of the tank vs. how much water is in the tank. In reality, just try it and see how it tows. If it tows ok fine, if not don't fill the tank. I would bet the manufacturer already considered that you will be towing with water in the tank. A hundred pds. here or there ain't going to really make or break you and you probably won't even notice it.