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Ah yes, I happened to read the MC6 Giant Carp last month while homebrewing a conversion of the MC2 Giant Catfish. The two have pretty similar stats, except the Catfish is a bit smaller and has poison spines.
I reckoned the Giant Catfish is Huge in 3E, so the Carp should be too. These fish are BIG. 20 to 30 feet is the size of a basking shark so they likely weigh between 4 and 15 tons.
I'd increase the Strength and drop the Dex, since (a) they're massive and (b) normal-sized Carp are fairly sluggish fish, and a giant version is likely to be slower.