I had an idea this morning for FT2.
In FT, the Golden Guppy is a legend and its powers are not known. So, in FT2, once all species are discovered and all the magic fish are found, have the program enable a new research line (like the research environment) but for researching the Golden Guppy's ability. maybe have it for level one, two, three, and four.
You could also have something like Research New Fish (this is after all of the 441 species are discovered), and this would do something like, in level one, it gives one new fish like, erm, Longfin Loach, and in level two, it gives two new fish to play with like, ah, er... Waveback Wrasse and Doublefin Catshark, or something like that.
Then each new fish could be backbred to the 21 original types to create even more new fish.
This is one way to satisfy the "more types of fish" that everyone seems to want!
And feel free to use the names that I came up with.
Rainbowfish