[willtel]

I have a small tank and use an air pump controlled by a float switch and a Nalgene carboy to top mine off.

I got the float switch from http://www.floatswitches.net/ and it is mounted in the tank and turns the air pump on and off. The air pump is connected by airline tubing to the carboy, the carboy has two tubes going into the lid, one short and one long tube that reaches to the bottom of the carboy. When the pump comes on it pushes air into the carboy and then the top off water is pushed out of the long tube and into the tank. Once the water level has risen enough the switch turns off and the water stops. This system works well for me because I have a small volume of water and the top off is added to the system very slowly which is important because I use kalk for all my top off water. Another advantage is that there is no submersible pump to worry about having to clean and the small airline tubing is very easy to plumb. I can post some pictures of my system later if you would like to see them.

Here is the carboy I use, they are available in many shapes and sizes.

 

[willtel]

I knew someone was going to make me drag out the camera

BTW, I assume the air pump has enough backward (for lack of a better term) resistance to prevent the hose with the water from just becoming a siphon and emptying in the sump?

I have the output tube placed above the water line so I don't have to worry about a siphon forming.

Also, when you refill the carboy do you disconnect the tubes from the lid?

I just hold the lid in place and spin the carboy. Mine only holds 2.5 gallons so it is pretty easy to move around and I have extra length on the tubes so I can do it on the floor with no trouble.

I have to admit the hardest part of the system was finding a container of decent size with a tight sealing lid that wouldn't give away under the air pressure. Once I found the Nalgene Labware website (http://www.nalgenelabware.com/) I tought I was home free but a new carboy like mine is about $85!! I found some used ones on eBay for $11 and I was all set.

Here are some pictures of my system.

This one shows the float switch and the hard output tube connected to my tank.

Here is is in action.

Here is a wider angle. You can see the kalk mixing under the tube.

Here is the carboy in my stand.

A shot of the tubes in the lid.

And some tank shots just because.

 

[willtel]

Yep I'm using the powerhead control for mine. I had to lenghthen the wires a little but it works great. Here is the pump I am using any decent size air pump should work ok.

A few things to keep in mind.
I don't leave mine plugged in right now because I don't trust the suction cup, it has come loose a few times and I'm planning to make a permanent acrylic mount for it.

Also be careful of where you put the float switch. Mine is in my refugium and I have heard of others putting them there and snails crawling onto them and causing a flood, or worse a nuking your tank with kalk. Several people have mounted them inside a drilled pill bottle or film container to protect the from snails. I plan to do this when I make my mount. There is another method of using two float switches, one mounted higher than the first to shut off the pump in case the lower one hangs in the on position. With the size of my tank and makeup water container I didn't think it was needed.

 

[willtel]

Like I said finding a decent carboy at a reasonable price was the hardest thing I had to do. One other place to try is home beer brewing stores. There are a few in town and they sell usually sell them. They may be glass but they would work the same.

The switch in the sump would be ideal. Just keep and eye on it when you first get it set up so you don't have any floods or other issues.