There are some fish that new hobbyists should avoid. The reasons vary, but there are certain species that are just not suitable residents for the community tank of an inexperienced fishkeeper.
Chinese Algae Eaters
These fish (often referred to simply as "algae eaters") are a very poor choice for any hobbyist, experienced or not. They are not particularly efficient algae eaters, and they tend to mature into nasty, territorial adults that often do not survive for very long, as they are prone to self-inflicted starvation.
For many years, goldfish have been popular pets for beginner fish hobbyists particularly for children. While goldfish are beautiful, intriguing fish, they do not belong in an average tropical tank setup. Goldfish grow to a relatively large size, and they produce too much waste to thrive in a regular tank situation.
Bettas, also called Siamese fighting fish, do not belong in a freshwater community tank. If you wish to keep one of these beautiful creatures, you will need to provide a separate aquarium just for them.
Mollies are often recommended as good beginner fish, but their sensitivity to water conditions makes them difficult to keep, and therefore, mollies should be reserved for more experienced hobbyists.
Other than cory or bristlenose catfish, these fish are poor choices for a home aquarium. Although they start out small, catfish can grow to be very large, anywhere from one to fifteen feet long. To keep any catfish species (other than the two exceptions) would require a large and complex setup, much more than the average new hobbyist is able to provide.
While there are several freshwater shark species, none of them are suitable for housing in a home aquarium. There are also several fish that have physical characteristics that give them a look similar to that of a shark and are often referred to as “sharks,” but they are not technically sharks, such as the red-tailed black shark and the rainbow shark. These fish are a poor choice for a novice aquarist, but if you decide to give them a try, keep only one specimen at a time. Multiple "shark" species generally will not get along well together.
As with sharks, there are some fish that are referred to as "eels" that technically are not eels. These fish are not recommended for beginner hobbyists. They tend to be difficult to keep healthy, and they are infamous escape artists. Many of these "eels" also grow to a large adult size that is not suitable for the average tank.