uh, yeah...pet shop people will tell you all sorts of things to sell you an animal. You can feed the canned stuff, but it's not nutritionally optional.
Well first line of questioning is always about the normal stuff - how old is your beardie, what's the housing set up (size of tank, lighting, etc.) so more info you provide about Fafnir the better.
Generally speaking, these lizards need to eat LIVE food (crickets & phoenix worms), especially when young. As babies (0-3 mos), they need to eat live food 2 to 3 times a day - as much as they can eat in a 10 minute period. Live food should be no bigger than the space between their eyes - I would error on the small side for that. As Juvies (3-12 mos) they need to eat 2X or 1X a day the live food. Translation of actual situation: our beardie is 5 mos old. He eats crickets in the AM and the PM, and eats upwards of 40-50 crickets each time, along with 12 phoenix worms. In addition to all that, you also need to include a diet of dark green leafy vegetables (such as collard greens or mustard greens) on a daily basis. As babies they need 80% protein, 20% veggies. As adults (~18 mos) they will need 20% protein and 80% veggies. So the cricket massacre subsides as they get older, but as babies they need a lot because they grow so much in first year of life. Here's a website for all the good veggie info:
http://www.beautifuldragons.503xtreme.com/Nutritionframeset.html
Crickets should be dusted with calcium power 5 days a week for babies (once a day), and 2-3 times a week for juvies, and 1-2 week for adults.
Don't feed them mealworms. Not a whole lot of nutritional value, and they can become impacted from the hard exoskeleton. Impaction can kill them.
Young dragons need to be
hydrated, as they get dehydrated easily. You can
bathe them 3-5 times a week, put eye drops of water on their nose and make them lick it off, keep water bowl (fresh) in their cage daily.
Please read this forum, tons of great information, lots of knowledgeable people on here (way more than me) and keep asking your questions...!