RO/DI or distilled water would improve the shelf life of your mix, but you're not storing it for years at a time. You are only making enough of this solution to last 20 days at a time. I somehow doubt that the minor pH difference (7.6 vs ideal 7.0) will matter much over that short a time period.
As for bacteria in the water, there really shouldn't be enough of them to matter unless something is rather wrong with your water supply. Even if you use distilled water that is 100% free of bacterial, you are going to end up getting a few in the container just from air contact (unless you mix in a clean room). Either way we are talking about trivial numbers, not billion of cells.