I just wanted to take a minute to tell you about a product that doesn't get as much attention as it deserves. I have been using the Coral Box A100 auto top off for a long time and it has worked flawlessly. It has an optical sensor and a backup float (protected with a clear cover). The best part is the price at only $70. Most ATO's are over $100. It is sensitive to water level changes and stops as soon as it reaches the set level (no minimum time).