I have been using this top off float switch for a couple of years and it has worked great.

(Click here for a picture of the Float Switch parts)

Take a 1 14/" PVC pipe cap and drill four small vent holes around the edge and a 3/8" hole in the center.
(Click here for a picture of the top cap.)
(Click here for a picture of the top cap - bottom view.)

The float switch is a Madison Miniature Vertical Liquid Level Switch model M8000.

Granger has them and their part number is 2A554. This switch will handle up to a 30 watt load. That is plenty for a powerhead to pump the makeup water or a solenoid valve on your RO/DI system.

It can be made to be normally open or normally closed by flipping the little float over. This means that this top-off can be used as a overflow shut off too.

Cut a short section of 1 1/4" PVC pipe.

Put the pipe cap on one end and a 1 1/4" PVC coupling on the other end.
(Click here for a picture of the bottom.)

I put a 1 14/" x 1" PVC reducer bushing with a 1" bulkhead strainer in it so that no snail or fish could go up in the tube and jam the float switch on.

You can get these bulkhead strainers from www.aquaticeco.com

I am not sure why I made this piece of PVC pipe so long because the float switch is tiny. It will work long or short.

You will need to make a bracket to hold it that fits your needs. I bent a piece of acrylic and drilled four holes in it so that I could tie wrap that float switch assembly to it.

I drilled and tapped two 1/4 20 holes in the back side and installed two nylon screws so that I could clamp it to the side of my tank.

My tank walls are 6" thick and that is why the bracket is so wide.

You can get nylon screws, nuts, wing nuts and washers from www.aquaticeco.com by the piece or from www.usplastic.com by the hundred.

You can get a heat tape from www.usplastic.com for bending up to 24" wide acrylic for $31.49 and 48" wide for $39.33.

This tape is not fast but it won't burn your acrylic. It works very well.

The float switch comes with 24" long wire leads on it. You will have to wire this up to an extension cord or an outlet box.

If you are not completely comfortable working with electricity, for your safety please let some who is take care of this part for you.