Copy and paste: taken from a previous post of mine
If you want to match your evaporation rate exactly, and never have to worry about adjusting your drip, all you need is a float valve and a tank. You could go buy a 10g, and take it to a glass shop and have them drill a small hole for the float valve at the bottom of this tank. Then just sit the bottom of this tank @ the top of your sump. This is a picture of mine. I had mine custom made out of acrylic @ a LFS here because I didn't have enough room for a 10g behind my tank. I believe the grand total for this project was $32, it works perfectly, and has made my life much easier. If I would have used a 10g it would have cost me $15. The adjustable float valve just sits down in my return pump chamber and it matches my evap rate perfectly. All I have to do is fill it up with Kalk/H2O every 2-3 days. I can also use the other two salt buckets for a combinded volume of 15g when I leave my place for a week or more.
Your salinity will never fluctuate with this set up. You will never have to worry about adjusting drip rates, overflowing, or not enough water. It replaces water at the exact same rate that it is evaporated, it is pretty much fool-proof. It is also perfect to drip kalk with.