I will chime in here. I have actually built several cheap auto-top offs that cost less that $15. All you need is a float switch and an extension cord. Use a float switch like this: http://www.grainger.com/Grainger/pro...mId=1611760105 And wire it in with an extension cord and plug your pump into that. You simply cut one of the wires and connect each end to the wires on the float switch. These switches can trigger low or high, depending upon which way you mount the cylinder (it flips over if you need). You can double your reliability by using two. I never had a single problem with mine. If anyone wants to build one, feel free to consult me for free advice.

However..... I will say that this time around I bought an osmolater. For something like top-off, I wanted to get the maximum reliability. The osmolater has two nice features: it has an IR sensor completely out of the water that backs up the float switch, *and* it has a time limiter that keeps it running for too long at once.