Sera double suction cup holder 1kpl E

A suction cup holder that grips a tube or device 23 millimetres in diameter.

What it is for: in the manufacturer's list this is the spare part for the sera aquarium heater thermostats. The heater is held against the tank glass by the holder, and the holder is in practice the part that tires first: the rubber hardens and stops gripping even though the device itself is perfectly fine.

The dimension decides: the holder is made for a 23 millimetre diameter. That is a figure to check rather than estimate – too loose a holder lets the device go, too tight a one will not fit at all.