These magnets make cleaning the aquarium glass easy. 

The cleaning surface of the inner part removes the algae layers. The surface of the outer magnet is covered with a particularly fine synthetic matt that protects the glass. The magnets have an oar that allows attaching a piece of string for retrieving it.

Available in:

S: 33 x 70 mm (1.3 x 2.8 in.) for glass thickness up to 6 mm (0.2 in.)
M: 42 x 92 mm (1.7 x 3.7 in.) for glass thickness up to 8 mm (0.3 in.)
L: 60 x 110 mm (2.4 x 4.4 in.) for glass thickness up to 10 - 12 mm (0.4 - 0.5 in.)