I had a serious growth of hydroids and had tried everything to kill them with no success. One day last summer, my chiller broke and my tank went up to around 92 degrees. It killed all my sps and fish, but also killed my hydroids and they have never been back. I suggest you remove a piece of rock with hydroids from your tank and place it in a container and run the temp up to 92 degrees. It worked for me and it is worth testing on a small piece of rock.
Jim
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