I use a 5gal container, a timer, and aqualifter pump. The auqalifter pump runs around $10 and uses airline tubing. It pumps a max of 3.5 gals a day. Perfect for top offs especially because you don't need to submerge the pump. This means if you dose kalk it won't deteriorate the pump as fast. I picked up a digital timer that has 24 on/off settings. Every 2 hours I have it come on for a minute. My tank loses 1/2 gal a day and 12 minutes of the pump running a day equals that amount. Later I plan to use a float switch, but this was fool proof and easy.