I would use the water from your 20g and mix with the new batch of saltwater to fill the new tank. I mix water from my display tank with some fresh saltwater everytime I setup my qt tank.
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RSM 250. Clownfish, Fox face, Blue tang, Yellow tang, Kole tang, Clown tang, Coral beauty angel, French Angel, splendid dottyback. CUC, softies, lps, sps.
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