I like it, maybe the produce will be random, or from the fish type with bigger amount (so like 1 Largemouth Bass+ 2 Carps = Carp produce), if there's one.
Maybe having 2 Fish Ponds which we can combine when empty, at Robin?
It could also be area based, so that each Pond can only have river, sea etc fish. As they need their own kind of water, of course.