If you want to keep a premixed salt storage tank then there is another fairly simple system you can employ.
- You essentially have two top off systems, one for fresh water and one for salt water.
- The fresh water top off works with a slow fill principal (slow pump or restricted solenoid valve).
- Saltwater top off works with a fast fill principal (large pump from storage container).
- Float switch for fresh water top off is located at standard height to maintain standard water level in sump. Should be controlled so water level remains fairly constant 24/7.
- Float switch for saltwater top off is located slightly below the float for fresh water.
- A pump on a timer pumps water out of the sump to a drain on a certain schedule to remove the desired amount of water.
Now the idea is the fresh water top off maintains the water level in the sump so the saltwater float isn't triggered from evaporation. However when the drain pump is activated the fresh water top off will be too slow to keep up so the salt flow will trigger and since it's fast fill, will top the tank back up before much fresh water is added. You'll also want to maintain the salinity of the storage tank slightly higher than the desired aquarium sanity as some extra fresh water will obviously be added every time saltwater is added.