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Yeah you can transfer it over. Once you fill up the 75 let it cycle then you can move the rock from the 55 into your 75. Im assuming you still have fish in the 55. Let the 75 cycle with the new LR you get after the tank is cycled you can just pull out the LR from the 55 and move it into the 75. After you have your rock work all set up you can then transfer your fish. I just did this with a 20g reef into a 55g. with no problems. HTH