When you have a pump that is too fast for the sump size, you will have to tee it off with a ball valve and let the water back into the sump or use that water in the ammonia tower. If you just add water to make up the water needed to circulate, you will over flow your sump if the power goes out. You should always do a shut down test to your system to make sure your sump do not overflow if the power goes off. The water will evaporate if you have it dripping into bio balls. The faster you have it flowing the faster it will evaporate. To minimize evaporation, you will have to put a cover on everything.