Here is a diagram of my top-off system. I got the idea off the Internet but I don't remember where. In the drawing on the left a siphon is started in the tube with the label "water out". Water will continue to siphon out of container "A" and air will be drawn into tube "B" until the water level in the sump rises enough to submerge the end of tube "B". At this point water will be drawn into tube "B" and rise until it is approximately at the same level as the water in container "A" (drawing on the right). This effectively stops any more air from entering container "A" and thus no more water can flow out of the siphon tube. The system will stay in this state until the water level in the sump drops enough to uncover the end of tube "B" which will allow air to rush in and the cycle will repeat. By adjusting the height of the end of tube "B" you can vary the water level in your sump. This system is very simple yet it can precisely maintain the water level.

I used a one-gallon clear plastic Rubbermaid storage jar called a "Collectibles" jar ($3.00) for container "A". You can use any container that has a tight sealing lid that can withstand the negative pressure created by the siphon without collapsing. Anything made out of glass would work, but in plastic something round is more likely to work, flat-sided containers will flex too much to give good results. For a five-gallon system a bottle from an office water cooler would be ideal. For tube "B" I used a piece of clear plastic tubing from PETCo ($2.00) that is about 3/4" diameter. An alternative that I came across since is a clear plastic fluorescent light tube protector from Home Depot, around $4.00 for an 8' length. You may be wondering why tube "B" needs to be such a large diameter, why not just use small diameter airline for tube "B"? In theory that should work but in practice when the end of the tube uncovers surface tension will keep a "plug" of water in the tubing. Also, tube "B" doesn’t need to be clear but it does make it easier to check that everything is in working order. The black part labeled "C" is just a piece of Plexiglas siliconed onto the end of the tube, it has a hole drilled in it to accept a piece of 3/16" clear rigid airline tubing. The rest of the plumbing is just more of the 3/16" rigid tubing and flexible airline, all sealed with silicone where necessary. Note that the siphon tube goes to the bottom of the container but the air inlet tube just penetrates the cover.

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If you have any questions or problems, feel free to send me an email:
s-pawlowskis@att.net