I have built EASY and reliable auto top offs for nanos before. I may use it for my 10 gallon now. They are INCREDIBLY easy to make!
Here's how. Get a cheap air pump. Buy a float switch. (The one I used is like this:
http://cgi.ebay.com/LIQUID-LEVEL-FLO...QQcmdZViewItem
I put it inside a medicine bottle (the bottle was drilled at the top for the switch, and drilled all around so that snails and such can't hang on it (can be mounted in display tank this way!).
You rig a 1 gallon bottle with screw cap (milk jug, maybe built out of
PVC, etc.) so that there is one airline in and one out. The in connects to the air pump, and only protrudes into the container about an inch. The other line goes all the way to the bottom. When the air pump turn on, it pushes the water out! NOW...you can't have the reservoir higher than the tank, or the first time it turns on, the resevoir will siphon out. The float switch is wired inline into one side of the cord of the air pump.
This is a BASIC description. It is really hard to describe. I'll find the link for the one I made. I hope I've said some things that make sense.
It is really simple, works well, and is pretty fool-proof.
-Jon
