If you have the aqualift pump and the Ultralife float switch then you only need a freshwater reserve tank to have an ATO system. That is exactly what I have and it works perfectly. The magnetic float is located in my sump. One thing I love about the Aqualifter is the the water is added very slowly to the sump with no adverse effects with changes in salinity. You can even add additives to the water reserve.
A = Aqualifter pump B = Ultralife Float
Freshwater reserve 5 gallon. The rigid tubing goes all the way to the bottom of the container.