I've never tested what happens when you change difficulty settings mid-game, so I can't really say how that would affect game play. This is what Arthur said about the difficulty settings for Plant Tycoon, and I'd imagine it's something similar for Fish Tycoon:

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The difficulty settings affect how accurately the plants are priced, how robust they are, and the effect of certain chemicals on them. It is easier to win on easy.

Since the genetic code that determines each plant is largly non-random, if you knew the 'recipe' for a magic plant, it would work the same on any difficulty setting.

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