This was very easy for me.
The model that the problem examples showed, a comma-separated list of all fish, seemed impractical right away -- I don't really think they expected you to use that unless you really didn't think about the problem much before implementing.
I went with something simpler, knowing that perhaps in Part 2, I might need to restructure the model if the problem changed enough. However, the model I used (and the logic) were entirely re-usable, so Part 2 was really just changing the number of days and renaming my part1 method to simulate.
I used RubyMine again, and this time it went more smoothly, so perhaps the issues I hit yesterday were resolved by making sure I had the right Ruby selected early on. Having said that, this problem was easy enough that I really didn't need much tooling support, so it might not be a great example.