well when you have saltwater its essential....the more water that evaporates the higher your salt concentration which will change your specific gravity. If you have an auto top off.....it will keep it at the specific gravity you want. My problem is......when I go on vacation I worry about it the whole time......because no one knows anything about my tank......so a ATO would be optimal. Check it out on melevs reef. He uses wrapping paper tubs. Probably what im going to use as well.