This is the begining of my continuous 90% automated water change system. Its going to service a 1000g system when everything is said and done. I say 90% because we haven't sorted out a salt delivery system beyond pouring the salt in manually.
I'm using a leg tank purchased locally, I think its 120g
Its mounted on a shelf in my fish room. RO water comes in and fills the leg tank where its heated and salt is added. Salt water is then pumped into a 10g tank situated under my display (on the other side of my fish room), and then 10g is pumped out of the display into the drain, and the fresh 10g is added.
Ideally we wanted something that would automatically add salt and measure for salinity to one of the 10g tanks, however I've had no luck sourcing anything that wouldn't be immediately corroded. I have however left ample access and opportunity for the system to be redesigned in the future if parts become available.