Amplexidiscus fenestrafer, the giant cup mushroom, will grow huge, and will eat fish.
Mine was 16inches in diameter, that is a yellow plastic ruler I put on it for the picture. Second picture is it balled up in feeding mode. I removed it from the tank because of space issues, it left behind tiny bits of foot on the rocks, 6 months later I had 5 of them 4inches in diameter. The large one ate a maroon clownfish in my propagation system after it was removed from the display.
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