Don't know if this solution will work for you, but you could get a 1/2 inch grommet, a 90 degree elbow, and transparent tubing (1/2 inch by whatever the height you need is). Drill a hole for the grommet toward the bottom of the rez. Stand the tube upright. Place a piece of PVC over it to keep light out and prevent algae. Just slip off the PVC tube when you want to check the level. It also doubles as a great drain tube when you turn it to the side or as an easy way to take a sample to test the solution without opening the rez. If the rez is also the pot the plant is in, this method is ideal so that you don't have to guess. It's what I do. I didn't come up with the idea. I got it from the GH water farm setup. The PVC cover was from the local hydro store. They do that on their water farms.
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