<a href=showthread.php?s=&postid=9491355#post9491355 target=_blank>Originally posted</a> by Amoore311
How about a DIY auto top off? They are really easy and cost maybe $20 at the most to set up.
You need:
Container to hold top off water (2 liter soda bottle works nicely)
Battery Powered Air Pump ($10 at petsmart)
Some Airline Tubing
Floatswitch from Floatswitches.net ($8 maybe cheaper)
First open up the air pump and splice the float switch lines into the lines for the air pump. Make sure you use a battery powered air pump since the float switch can't handle the current comming out of your wall socket.
Once thats all set drill two holes in the cap of your soda bottle/container. If you make the holes just small enough that you have to pinch the airline tubing to make it fit through the holes you won't need to silicone or seal around them.
the piece of tubing going from the container to the air pump should no go below the water line in the container. This is to prevent a syphon once the pump shuts off. The other line should go from the bottom of the container to the tank. Again make sure this line is above the water line in the fish tank to prevent a syphon.
Once it's all set up mount your switch in the tank at the desired water level and you're all set.
The reason it works is b/c the air pump pressurizes the container. The easiest way for that pressure to be relieved is through the airline tubing running to your tank.
My pump takes 1 C battery and has been using the same battery for about a year.