"Purified drinking water" is not always the same as pure H2O - often times it is just treated to neutralize whats bad for you. How does it taste? Pure H2O out if a rodi actually doesn't take all that good - its the minerals and stuff that adds the taste. I would still check it with a TDS meter to see if its a 0 reading.Tada, I use bottled water. 30 cents a pop. Pure pure pure!
It's likely better than tap but the issue is you don't know what else is in it. While in these concentrates it might be OK but remember your salt mix also adds minerals and trace elements, would it still be OK after yiu mix in the salt is the question mark.
Plus microplastics...in a closed system like tank it will accumulate fast.