I lurk mostly, but I have been watching this thread because I have an RODI tank in my basement. It is a 250 gallon tank measuring 7' tall x 3' in diameter and stays at a constant temp in the low 70's. I use it for water changes and emergency water needs. I have floats with RODI lines not only going to this tank, but I have lines going to my DT for top off, a Brute salt mixing barrel and a line going to a new tank. I have had this RODI water tank for several years and have never needed to clean it. I do 40 gallon water changes monthly on a 75 gallon DT then fill the Brute from the 250 gal container then the RODI system fills the tank back up.
I have a question...how do you know that the calcium, alkalinity and mag drops if the salt water sets over an extended period of time? Have you measured it? If the barrel is covered where does it go? After I do a water change I mix up a new batch of salt water to always have it available in case of a critical need. I keep it heated to 79 degrees and use an air pump and circulation pump constantly. I have done this for 15 years and have never had a parameter problem with my DT. I have never tested my salt mixture after it has sat for several so I can't say for sure the calcium, etc has dropped or not. I am curious???