It isn't fun going to my garden and finding the pump grinding away in a mere 1-2 inches of water. When the water in the sump gets that low, you no longer get adequate water circulation in the system. Plus, it's hard on the pump.
I spent hours trying to figure out how to modify a standard toilet float valve to add water to my sump to replace evaporation. I may yet figure it out. Or you may figure it out and explain it to me. But using toilet bowl hardware to regulate my water levels is still a mystery.
Luckily, I have a smart mother who has lived much of her life in desert conditions. When I explained the problem to her, she said, “You need a float valve like they have in swamp coolers, to replace the water that evaporates.”
I live in a humid climate where there are no swamp coolers. So no one sells the bits to replace swamp cooler parts. The big hardware stores don’t even list these parts when you do online searches, because no one in a moist climate would need to buy such a thing, and they stock them in dry climates in such quantity that there’s no reason to allow dry climate folks to buy them online.
There’s a flaw in there, somewhere. It turns out it is possible to buy such bits via a site like eBay, once you know what you’re looking for. Better yet, have a friend in a dry climate buy the bits and ship them. I assume fine aquaponics stores everywhere will start stocking them soon, as well. In the mean time, you can search for "aquarium float valve" on a site like eBay and find something to do the trick. The float valve itself will run you about $15. The tubing and bibb to connect to your system will be additional.
The way it works is the plastic bulb floats on the top of the water. As long as the water is high enough, the float keeps the valve closed. But once the water level drops, water will flow in (from a special bibb and tubing you connect to your faucet or some other water source, like a water barrel). Easy peasy.