Most tanks do shift a little from day to night. A reverse photoperiod lighted sump may help.
I think you might want to buffer your RODI water, and top-off water. Test the water *before* you add it to your tank. Check the Sg, Ph, Ca, Alk, even PO4 if necessary.
There are 2 part additives that are easier to use, and probably more stable for a 12 gallon tank.
There are really good fourms on RC in the Chemistry section, I have found all of them very helpful.
Sorry to hear a few of your fish died.