Murray Hallam shows a cool doo-hicky for automatically topping up a tank in his video Aquaponcs: The First 12 Months. Looked like an old-fashioned toilet bulb, but mounted differently from a toilet bulb.
I went to my local hardware stores and failed to find anything that seemed to work "right." So I came up with my 5-gallon water-cooler approach to keeping my tank full.
Except that it's high summer now. I'm having to fill the 5-gallon jug every couple of days, instead of once a week or so. It's still less water than I would use to water this many plants in a conventional garden, but it's irritating.
Then tonight I was talking to my Mom on the phone. Turns out the float valve Murray had in his video is a standard component in evaporative coolers (aka swamp coolers). The reason I'd been unable to find the piece is that swamp coolers don't work in Virginia because water doesn't evaporate terribly well in Virginia's humidity. Therefore big hardware stores in Virginia don't stock parts for evaporative coolers. They don't even let people in Virginia (and presumably other humid regions) find these parts on the websites, since it isn't a part they bother stocking in their online shipping depots.
Now that I know what to call the thing, I could order one on eBay. Sweetness. Even better, my Mom is willing to jot down to her local hardware store and pick one up for me. In turn, she's asked me for a bag of my bacteria-impregnated rocks, to help get her aquaponics system started. Kind of like how neighbors of yore used to lend each other a cup of sourdough starter.
So there'll be one "Dial Evaporative Cooler 1/4" Brass Float valve " headed from Utah to Virginia, and a pint of damp hydroton headed from Virginia to Utah.
In the mean time, you now know what to look for on eBay, should you want to set up such an auto top-off system for yourself.