If you have a large tank, one that is over one hundred gallons, you will most likely want to look at some larger fish. There are some great large fish on the market today, some which will grow to over a foot in length and eat right out of your hand! Keep in mind that if you are going to buy a fish that grows over a foot, your tank should be at least eighteen inches wide, to give your fish the ability to turn around easily.
There are a couple of large fish that prefer to live solo. They are not compatible with other fish, and will attack and kill their tank mates, unless they are in a huge tank. The Red Devil falls into this category, as they will claim at least one hundred gallons as their personal territory. Shankeheads also do not do well with other fish. Snakeheads are very strong, and some tank owners say they have broken the glass of their aquariums.
Oscars are probably the most popular of the bigger aquarium fish. Oscars have the reputation of being violent and aggressive, perhaps because they prefer live food, such as feeder goldfish. They are not aggressive to fish that are their own size, and often are the recipients of aggression from other fish. Oscars are in the cichlid family.
Arowanas are an interesting looking fish, but they get huge! They can grow to lengths of three feet long. They are easily startled, and will fly out of the tank if they panic, so make sure you have a well-sealed tank. They are peaceful fish when kept with fish close to their own size. Clown Knives are another peaceful big fish. They are prone to attack by fish that are more aggressive. Clown Knives are picky eaters, preferring live food. True Gouramis are another very peaceful big fish.
Pacus are another large fish, and can grow to be over fifty pounds! They are closely related to the piranha, but are vegetarians mostly. They will eat anything that fits into their mouths, so keep small fish away!
If you have a huge tank, over 1000 gallons, you can own a Redtail Catfish. Redtail Cats will eat any fish that fits into their mouths. They will get sick with a skin condition that can kill them if you attempt to keep them in a tank that is too small for them.
Shovelnose Catfish grow from about eighteen inches to three feet. These cats also have large mouths, and can actually swallow fish that are up to half of their own size. These large fish tend to be very hardy.
Tinfoil Barbs are very fast fish, and therefore can sometimes be mixed with more aggressive fish. These tend to be hardy fish.
Iridescent Sharks are a peaceful fish that grows to about three feet. They are often attacked, even by peaceful fish, and will loose their eyes and fins. These fish like to move, and need plenty of room to swim.
Silver Dollars are another active fish and some species fall into the large fish category, like the Red Hook, which grows to over twelve inches. These fish are active and mix well with other fish. Keep in mind that there are some Silver Dollars that do not grow large.
Finally, if you are looking for an algae eater, go with the Lyretail Plecostomus. The Lyretail Pleco can grow to be several feet long. They have a heavy body armor that makes them almost impossible to attack.