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It will depend on what's in your 75, and how the 125 is set up. You could maybe keep a larger angelfish that doesn't get over a foot long (maybe a majestic or a pair of Genicanthus). I don't think I'd keep three tangs in a tank that size, maybe two of the smaller, less aggressive species (NOT sohal, naso, vlamingi, etc.)
What do you want to keep in the tank? If you post a stock list, it would be easier to respond with useful information.
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