ATO nonstop - Can't figure it out!!??

I actually do have a mark on my sump for the water level. That was how I knew it was dropping and/or everything is normal. Thanks!

 

You know, I thought the same thing at one point. But I dismissed it because I thought the Tunze ATO had a mechanism to prevent a back siphon. My ATO tube was in fact below the water line while I was having the problem....specifically in my baffle zone. A siphon could totally explain the frequent water drop, and explain why it continued to drop only in the baffles, and then suddenly stop (once the tube was above the water). When I put everything back in I positioned the ATO tube above the water line (for whatever reason). And poof.....everything miraculously started working again. I think you figured out the issue!! Thank you!!

 

Check the salinity in your ATO resevoir. If you have been adding salt to your sump to bring your SG up, and your sump has indeed been siphoning back to your freswater resevoir, then you will be topping up with saltwater.