8 weeks in your tank and the tank has been set up for 11 weeks? So that means you put the fish in the tank after 3 weeks of setting up the tank, correct? That may be too soon to add a fish.
Check your ammonia. High level of ammonia may cause rot fin. Some other things may also cause rot fin, like bacteria or parasite like ich. the latter two may not be as deadly as ammonia.
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