How to automatic mixing water

[donlite]

This is a little of base but I am looking at having 2- 20 gallon plastic containers one with fresh water and one with high concentration of salt. When water evaporation is detected it acts as a auto top off system filling it with fresh water. When it needs a small water change using 2 salinity meters one in the tank and one detecting the water being mixed it can perform water changes. The system is not that simple but at the same time there is parts that you can purchase from robotic sites that are cheap and easy to hook up to your lap top. Let me know if this already exists and what you think, Don