You do not need to lower ph but need to make it stable. Also be sure to remineralize your RO
For your aging barrel you can get it up off the cold concrete and wrap it with insulation.
Though sulfur reactors are proven with results it will not be a substitute for water changes. It may actually be problematic if it is causing ph fluctuation.
How many juvies and what size are you planning? They will quickly outgrow a 40 so do you have plans for a larger tank?