I know I just joined this site,
I was having the same problem trying to figure out how to keep my sump from dropping to the point of needing to add water every few days. I was looking at my dog water dish with an inverted bottle attached that keeps the dish full. As soon as the water level in the dish reaches the bottom of the bottle, the bubbling (and auto-filling) stops by itself. I decided to try a similar system on my sump. I also brew beer which gave me some supplies to work with. Supplies needed: glass bottle (glass doesn't give at all), bottle stopper (one with a hole in the middle that a tube can be placed inside and a perfect seal is made on the glass and the tube), aquarium tubing that fits in the bottle stopper. These supplies are available at your local brew store, the bottle stopper is for inserting a pressure check valve for letting co2 escape for brewing.
Invert the glass bottle with the stopper in and the (.25 inch) aquarium tubing inserted into the hole in the stopper. Adjust the level of the tube to the level you want your sump to stay at. As water evaporates, it will drop the water level in the sump. This in turn will allow air into the tube, then the glass bottle, and allow only enough water to enter the sump to keep the level where the tube enters the water in the sump. A bracket can be made to hold the glass bottle, or the bottle can be set on the gap of glass or acrylic where the return pump is located (put the bottle here or it won't work).
Cheers and good luck!
Cost was less than 10 bucks!
