Not ALL pet stores are run poorly, and have bad advice. There are a few that actually care about the animals they sell. Plus, not everyone has the ability to buy over the internet. I have bought many of my animals from pet shops, and one in particular has sold excellent quality animals to me. I got my Cranwell's Horned Frog from that petstore, and he is in very good health because the owner of the store had him in the shop for 2 wks before he sold him so his health could be monitored. He was fed crickets with dusting powder to give him a healthy meal, and had the proper environment in the holding tank. Don't immediatly bash all petstores until you've seen them in person, and know the quality of care the animals get while there. Not all of us can get animals over the internet.
But I can agree on one point, though. A Horned Frog shouldn't be fed fish or mice. Crickets are the proper staple diet, and extras to add variety are waxworms, butterworms, earthworms, cockroaches(when the frog is bigger), spiders, ants, moths, flies, and strips of beefheart if you can get them to take it. If it's been eating a steady diet of fish, I wouldn't buy it because it might have a few health problems that possibly can't be remedied. Find a reputable dealer so you have a chance at getting a healthy Horned Frog.