Carol, I reread the thread and missed one of your questions.
The RO/DI water will supply the bottom barrel for mixing and a reservoir behind the sump for ATO. It will be a MaxiJet 600, controlled by a 12 volt relay, switched by two float switches. One switch will be a low level normally off switch. The other will be a high level normally on switch. Either switch being in the wrong position will turn the system off. Redundant protection. If all that fails, I have a 1 inch bulkhead plumbed from the sump to the outside for overflow protection. It's about 1.5 inches below the top of the sump.