spotted puffer fish

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A while back i purchased two spotted puffer fish. I researched alot about them and made the water temperature perfect and had a very nice aquarium for them with a pump. After about a week one of them died. We ended up buying two more along with a sucker fish. Not long after that 2 more fish died. Then a week later the sucker fish and the last puffer died. Has anyone else had this problem? I know i did everything right but i don't know why they kept dying.