If you have the chamber levels set correctly, you have the sensor correctly mounted in the return chamber and you followed the directions for initiation then send it back. The Tunze should work well but if not I'm using a JBJ Nano ATO on a Waterbox 15 and it has been flawless for the last 8 months (when I installed it).
The biggest issue I have seen with Waterboxes is that people tend to adjust the display height by adding water until the display level is where they want it by overfilling the sump so that all of the chambers are the same height as the display. For a Waterbox 10/15/20/25 the chambers should be at three different levels while running the return pump; 1st chamber at about the height of the sock holder--it will vary a little based in media type and cleanliness, second chamber at height of the lower baffle in the third chamber and the third chamber at or below the same baffle--the third chamber is the only one that should change height with evaporation. If they are the same height as the display then the water parameters fluctuate too much because the entire tank has to evaporate to the ATO turn-on point, usually about an inch. In actual practice it usually evaporates until the entire tank comes down to the standard overflow height and then just the second and third chambers continue to go down until the turn-on point. Either way it has to replace an inch to the whole tank instead of just an inch to the return chamber.
That forces the ATO to run too long which defeats most of the fail-safes used on small ATOs (usually a time out function). It also allow far more S.G. swing due to the larger volume of water that needs to be replaced.
Just a possibility... Good luck.
