Found this useful website that breaks down the types of fish in different classes:
A - Fish that should never be kept in captivity, period.
B - Fish that may be kept successfully, but ONLY in public aquaria that can supply their specialized needs.
C - Fish that are often kept successfully in larger reef aquaria, but usually do not survive in fish-only aquariums. This is commonly due to specialized diet requirements that reef aquaria provide, in the way of naturally occurring micro-crustacean populations, but in some cases, may simply be due to the more complete natural ecosystem found in reef tanks.
D - Fish that are now bred and raised in captivity. Tank-raised specimens of these species will do well in aquariums, whereas wild-caught may not.
E - Fish that are almost always caught with Cyanide or drugs, and whose purchase supports this horrible practice.
F - While not impossible to keep in a home aquarium, requires either a specialized diet and/or habitat that make it impossible to keep with the majority of other species. Many should be kept in a dedicated tank. This classification is also used with very aggressive fish, or those that can eat larger fish, requiring special consideration when choosing tank mates.
G - Venomous. Exercise extreme caution when catching, or when cleaning the aquarium, and do not keep if you have small children.
H - Fish that should not be sold or purchased because they are rare. We felt the need to emphasize this one. Imagine public reaction to our hobby if it were discovered and published that the aquarium trade was responsible for the extinction of a species.
Thanks to:
http://www.aquariumpros.com/articles/badfishlist.shtml