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This person knows fish. Please, please, please do some research. Hardly anything will be good for a 5 gallon. I have had a goldfish grow to 7-8 inches long and I am 28 and got it in 1st grade.
Betta would be my only choice if it was me.
1 inch of (full grown) fish to 1 gallon is the rule of thumb, but all rules of thumbs are not perfect and do not apply to every scenerio.