If your not doing a full flush of the fluid, they do make additives for the system. Generally bringing down the temps by a few degrees with some success, and no adverse side effects that I have seen thus far. I recommend topping off with something like water wetter. But if freezing temperatures are not an issue for you geographically, you can top off with distilled water. Only distilled! Any other waters have minerals etc that will affect your cooling system over time. Best of luck.