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If you're on monster, it isn't possible, due to not being able to cross-breed piranha and butterflyfish for Blue Tang due to a environment conflict, in the version they are using. Otherwise:
Nope the latest version of Monster is using Mariculture 1.2.0e, that specific combo was fixed in Mariculture 1.2.0h (with another similiar messup being fixed in i (not being able to cross-breed to get enderfish))