A member of the Cichlidae Family. South American in origin, they are an aggressive fish. This one is full size, 5 inches in length, but can easily fend off larger fish such as a Red Devil, Oscar, etc. In the tank with it was a Clown Knife Fish, (photo below) which was 20 inches in length. The knife fish are not normally aggressive, but it learned to be a bit more aggressive and defend itself against this nasty little fish. Now they live quite happily in the same 60 gallon aquarium. 

The Blue Acara (Aequidens Pulcher)

The Clown Knife Fish is native to Asia, Thailand, India, Cambodia. They can grow quite large, this one is 20 inches and growing, in a 60 gallon aquarium. They like their privacy, do not move around much. I have sometimes thought it expired, it was so still. Unfortunately, it was curious about the camera, and started swimming about when I photographed it. Accounting for the slight blurring.  

Clown Knife Fish (Notoperus Chitala)

Armored Catfish, member of the Loricariidae Fish Family and originally from South America. These quaint algae eating fish were referred to by a noted Aquarist as "Worthless Ugly Beasts" About sums it up, except that they voraciously eat the algae from the walls of the aquarium. Unfortunately, they eat everything else in sight, being bottom feeders. They generate their weight in waste (or so it seams). They can also tolerate most aggressive fish, since they are covered by scale that is very hard, and impervious to another fish bite. They can be intimidated by very aggressive fish, and I had a red devil kill one by biting it every time it moved. The red devil could not inflict any damage to the Pleco, but it did intimidate it and it did not move. Not even to eat, and subsequently died. The Pleco in the photo is in the tank with the clown knife and Blue Acara. Note the Blue Acara in the photo, looks like a minnow. That is because the Pleco is 27 inches in length. A Beast.  

A Plecostomus (Hypostomus Plecostomus)