One difference between a scientist and an engineer is that an engineer looks for things that work as he wants them to, and only secondarily wonders about the science behind that. A scientist looks at the science first and rarely ponders whether something will work ok. (You can probably guess that I am a retired engineer.) So, something that works very well is removing half of more of the water from the aquarium, adding a dose of Prime to the remaining water that is a full tank dosage, then adding tap water to refill the tank. It also works very well to use your finger as a thermometer to match the replacement water's temperature to the tank water temperature. I used to use just cold water for water changes, but when I did that I limited my changes to about 25% of the tank volume. That worked well too. I don't know why this technique works, but I assume it is because fish are accustomed to sudden changes in water conditions that occur when it rains hard.

We also wait until the tank is refilled with tap water before we add a GH booster, if needed, and that has to be causing the fish to go from good GH to low GH and back to good GH very rapidly. Again I don't know why it doesn't harm the fish.