One of my mollys fins have stuck up and its become very fat what could this be?

Answers:
http://www.fishjunkies.com/diseases/drop...
Your fish has dropsy. It is almost always fatal. Check out this link to prevent further problems. Email me if you have any questions.

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well,
we have Molly's too,
and they all got really fat,
so ether they eat to much or its just the way they are