Downsides to Koi: not inherently smart as animals go, you'd have to make a lot of modifications.

Upsides: external eggs produced in large quantity, cheap to raise in lab conditions, lower ethical standard for experimentation than primates or dogs for example, there are already companies producing genetically modified koi as decorative aquarium pets so there's already an established industry pipeline for working with them, their skulls grow during their lifetime rather than primarily during fetal development & infancy like mammals which means it would be easier to give them bigger brains, lower chance of premature shutdown of program due to public backlash because they seem relatively low ethical concern and low threat (relatively passive natures, lack of object manipulation ability), social enough that they can be kept in groups without trouble.

The advantage of octopuses are that they are smarter to start with, and already have natural behaviors relevant to higher level thought like object manipulation, tool use, active large-prey hunting, and deception.

Disadvantages of octopuses relative to koi: harder to raise in a lab, harder to raise a bunch of them together, harder to keep imprisoned once smarter (even current octopuses are escape artists, the smarter each generation gets the worse this problem will become), octopuses die after mating so its harder to maintain a breeding population of the best-successes-so-far.

Advantages both have over mammals: external eggs, lower ethical concern (at least at first), high numbers of offspring-per-parent possible, lack of skull restriction to overcome, low risk of terrestrial habitat inflitration if facility escape occurs (easier to staff a facility not connected to natural water sources than a facility not connected to landmass).

Edit for clarity on resolution conditions: 

Will this market be resolved based on "legally recognized as a person by the USA or EU based on an increase in cognitive ability" like the other one?

 I would also resolve it upon finding strong evidence that I personally believed, so a somewhat lower standard than the other one. It would be something like "the fish can reliably pass IQ-type abstract puzzle tests & show proficiency with complex communication that puts it in a category above the smartest known non-human primates. Also, it has an approximate neuron count such that this seems plausible via current neuroscience understanding of information processing in vertebrates."

Note: I have subsidized this market and also expressed an implausibly strong opinion in order to kickstart gathering opinions on the matter. I am hopeful this market will draw interesting comments over the next 18 years. Perhaps indeed, it might motivate some bettor to research and divulge information about this being about to conclusively succeed or fail.