Hi Fishmanic. Your fish may simply be scared or uncomfortable for some reason. Fish have clamped fins when they're either scared or sick. But if your fish is eating and otherwise swimming normally, then maybe it's just scared. Observe it closely for any abnormal behaviors and any abnormal changes on its body. If none of these occur it's probably just scared. How long have you had this fish? What are your water parameters?
one question...what type of setting do you have for your red tailed shark??
as in tank gallons....
what other fish you got...
and so on...