H'm, I didn't see your post #16 previously when I added my #17...oh well, here we are.
You're doing a 25% water change, so that will be "x" gallons [checked through this thread and couldn't find a mention of the tank size, hence the "x"]. I would mix it 1/3 mineral, 2/3 pure; you could do this in pails, then pour in. [How do you warm the replacement water?]
Three days later, do another identical change. Then a third, next week, same proportions. Keep going until you are where you want to be in the tank water (GH at 4 dGH, KH 2-3 dKH, or whatever)--I cannot say how many changes this might take, could be a couple or a few more. This may seem overkill, but I like suggesting a safer method to others whose level of experience I do not know. If I were doing something like this, it would be done quicker, because I can easily read fish reactions and respond accordingly. I raised the GH alone (not KH or pH) in a couple of my tanks from 0 to 5, 6 and 7 dGH, for 2-3 years. This was for the plants, another story, but my 50% water changes when the GH would suddenly drop in half and then rise back overnight did not seem to cause any problems for the fish. I stopped this in 2015 and still have the same fish. But again, I was careful to monitor the fish during the changes and after, just in case.
Byron.