If you are doing a water change you are taking out water as well as putting in water. If you were just adding water yes it would overflow, but since you are taking out the same amount of water as you are putting in it will not overflow. When I do water changes I take out 1 bucket of water then add in a bucket of water and so on till I've changed the amount of water needed. My sump is only a 33g, so if I were to take out to much water before adding new water my sump pump would end up running dry.
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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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