The magnet holder for the Tunze 3155 ATO bumps up against the bottom of the plastic rim in a standard tank when trying to raise the optical sensor/water level high enough to prevent HOB filters from excessively splashing and making noise. This adapter hooks to the plastic rim with nylon screws. The two plastic tabs that hold the emergency shut off float switch and the optical sensor are highly adjustable up and down and will give you greater ability to set your water level.
Printed in black red or blue reef safe ABS plastic. No metal or magnets to rust!
Available in 3 widths for different sized plastic rims (red line in the second photo). Larger stock tanks have a wider plastic rim/brace. Measure across the brace, including the inner lip to find which adapter will slip over the whole plastic tank rim.