This is a small mount that attaches via four strong neodymium magnets to the side of your tank or sump can hold sensors. Works great for the 11mm IceCap ATO sensors.

Holds up to 13mm (1/2") thickness.

From my magnet supplier:

These great magnets are coated with a durable layer of plastic. The 1/2" x 1/4" dimensions are the finished dimensions of the plastic coating. These discs can be slammed together many times without damage. They won't mark or scratch surfaces they come in contact with. They are also completely weatherproof.

The magnets are exposed for easy observation.

I wish I have had a 10year history with these magnets before incorporating them in my products and allowing people to use them. So far, my 6 months of use has been great and I see no wear in the ones I have. Please investigate the magnets while cleaning the sump.