Welcome 
I don't know about the worm (maybe an intestinal parasite?) but the lethargy of your fish concern me. Also - a LOT of pet stores give inadequate advice. What kind of fish do you have? In what size tank? At what temp? The water chem might be off as well, so test it for us and post your parameters. All of these factors (and more) could really affect how your fish are behaving and their overall health.
I don't know about the worm (maybe an intestinal parasite?) but the lethargy of your fish concern me. Also - a LOT of pet stores give inadequate advice. What kind of fish do you have? In what size tank? At what temp? The water chem might be off as well, so test it for us and post your parameters. All of these factors (and more) could really affect how your fish are behaving and their overall health.
