Top up system

[TheReefGeek]

1. Pick your container - as big as can fit so you dont have to fill it often
2. Get any sized powerhead, and tubing to fit on the end of the powerhead up to your tank where you want water to enter
3. Now it gets more tricky. There are retail setups from companies such as Spectrapure and and Reefanatic, check out http://www.saltycritter.com/waterlevel.htm
I would not use the suction cups though, they will fail eventually and you will flood.
I would find a better way to secure the float switches. You can also build your own system from scratch, buying the float swiches and a 12v relay and power cord and doing some basic wiring.