to give you help incase you need it in the future, most pet shops will allways give you advice but come on ...the internet can tell it all...
your pi ran ha
need tetricyclin or myacin possible.
take video of fish using phone, pet shop not store, or emergency vets.
its possible that is paracites, in fact probable! gl
btw I own 2 of the largest in captivity, bout a foot long.