My LFS told me that "expert" mainly refers to how hard it is to get the fish to eat. He said that assuming you have a stable tank, then getting a fish to eat is the hard part. I currently have a Blue Hippo in QT and it hides about 85% of the time but goes crazy when I feed the tank. I see it come out more and more each day though. I'm sure there are people out there with a lot more experience with this than I but wanted to pass on the info in case that helps at all.
While getting an animal to eat is important, there's a lot more that goes into "expert" care than getting them to eat.
Octopuses are great examples. They're usually listed as "expert only", but it's really easy to get them to eat: just give them live fiddler crabs or crayfish. You've likely already spotted the problem with that: live foods are either difficult or expensive to obtain (or maybe both). Then you have the issue with filtering the tank. Because they eat so much, their tanks get filthy quickly if you're not careful. There's also the issue of them escaping. They also ink when startled, which, if not cleaned out quickly, will kill them. There can also be behavioral problems. I work part-time at the National Aquarium in Baltimore. Recently, we moved our Giant Pacific octopus off exhibit because we're renovating her enclosure. She was not happy with the lighting in her temporary tank and would frequently block the overflow in protest, which caused the tank to flood. We figured out that we needed to black out the lights on top of her tank and she's been happier.
Rays and skates are another "expert only" animal. Again, feeding them is really easy. At the Aquarium, we actually use whole mackerel and shrimp bought from a seafood distributor. Chop it up and feed it, not much more difficult than that. Again, though, there are certain challenges that can be problematic to less experienced aquarists. First, rays are sensitive to copper, which means treating illness is really challenging. They need a lot of space and aggression can become a problem quickly if they're in contact with their own specie. You need really robust filtration because again, these animals produce a lot of waste.
I agree that the label "expert" or "expert only" can be a little misleading, but it's not just about feeding. There are lots of other considerations to take into account. While I don't think I would classify a tang as "expert only," you need to do your homework if you want to get one. They have pretty specific requirements and can be a handful if you aren't pretty familiar with quarantining and aquarium husbandry.
