If you've got 75 gallons to work with, get yourself a snapping turtle. No matter how small they are when you get them and how long you have them, they never tame. It's like keeping a slow, angry dinosaur. Fast as hell when they're eating.
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My buddy said when he dies, he doesn't want to be buried or cremated, he wants to be eaten by his turtle.
Swimming with a crocodile would be cool.
Those Green Sunfish are the best IMO. They are aggressive, and don't get too big. Oscar's are boring I like the Native fish better. Plus add a big crawdad or two. They are funny when the catch a minnow. Plus its cool watching them molt. Creek chubs eat minnows also and I like them in there as well.