How to automatic mixing water

[Coleus]

Well, i am thinking of a process to mixing my fresh salt water automatically so i can have auto waterchange.

I have salinity probe, float switch and outlet controls so i can turn on and off outlet when it meet the salinity. My only puzzle is how i can find some kind of rotating drum (just like a fish feeder), that can slowly pour the salt into the water bucket until it reach the salinity level. Anyone know one that is bigger enough to do the job?