I don't know what the limit in the Palm version of Fish Tycoon is because I haven't tried to test it, but I have over $7 million in my PPC Fish Tycoon game right now.

There isn't a way in the game to tell which fish you've discovered and which you haven't - only the total number of species you've discovered. That's why I had to track the fishies manually when I was testing the graphics on each fish to make sure that I got them all.