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You would keep nitrates ,he would have enought with the forty gallon and he is adding more rock and sand so it would be like a big water change. I change tanks alot and dont lose a thing and I never herd of keeping old water. That is my advice
Why would you keep old water that is a no no, make up new water and take the water from the 40 so that would be a 30 gallon new water change minus more rock and sand I did it alot and dont lose a thing