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When you change 25% of the water you are removing 25% of the water in the tank. When you do another 25% change you are removing 25% of the 75% that was left in the tank. Etc. So, no, 4 - 25% changes wont remove 100% of the old water, just 68% of it. But, your plan is still a good one, doing 25% water changes until almost all of the old water is removed, about 10 changes to remove 95% of the old water.

I would do both tanks at once, since it will be the least disruptive to your routines.

Whether or not you need to add any GH builder or Magnesium sulfate or Calcium sulfate depends on what is in your tap water now.
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