If like me you are fed up with the suction caps falling off the walls of your aquarium here's a possible solution. This clamp uses magnetism to hold your heater or pipes to the wall. It uses 8x3mm neodymium magnets placed on both the inside and outside face of the aquarium glass.
Neodymium are particularly susceptible to corrosion, so it is recommended that the part that is immersed gets some duct tape stuck to the bottom and trimmed around the edges. The magnets have a chrome protective coating, but I would rather not take chances with my fish.
This particular design is for 21mm diameter heaters but it can also be used to hold pipes in place using small zip ties.
If you would like to have the design for a different diameter, please leave a comment and I will see if I can accommodate your requirements. All I ask in return is for you to post a make of the final print. :)