You shouldn't have a turtle in a tank with puffers. Puffers are toxic fish. Not to mention that any fish in the same tank as a turtle could be eaten. So say you did add a turtle, and the turtle ate one or all of your puffers, you turtle would probably die because puffers have toxins in their organs. You'd be left with an empty tank.
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Phishie
Site Coordinator

"An ounce of prevention is worth a pound of cure."