The suckermouth catfish, more often called the pleco, can share an aquarium with convict cichlids. But since various pleco species appear in pet shops, you must be careful to select an appropriate pleco in terms of demeanor and size. That little pleco from the pet shop can grow up to be a big fish in too small a tank.
Most pet shops sell the common pleco at a few inches long. However, this species can grow to more than 2 feet long. The pleco typically has a peaceful demeanor, but a specimen that size is not an appropriate tank mate for convict cichlids, which max out at around 6 inches. However, some plecos, like the busy nose pleco, stay smaller. You should select a pleco roughly the same size as your adult convict cichlids.
Convict cichlids can get aggressive. These cichlids get most pugnacious when breeding, defending their nests from all other fish. However, the pleco is a very robust fish. In a large enough tank, pleco species can share an aquarium. To ensure that your pleco does not receive too severe a beating, make sure that any tank these two kinds of fish share is large and that it contains hiding places and decorations that break up the lines of sight. If a pleco has means of avoiding the convicts, they can share a tank.
Most plecos have a peaceful reputation. However, a few species of pleco can get extremely belligerent. Specifically, panques, including the blue-eyed panque and the royal panque, become downright vicious. These species grow a bit larger than convict cichlids. Their aggression -- coupled with the armored pleco body shape -- makes them a serious threat to any other fish in the tank. Never try to keep panque species with convict cichlids. With most species of pleco -- even peaceful species -- you should only keep one individual specimen. Most species of pleco tend to be very aggressive towards their own, even those who don't normally bother other fish.
Other Tank Mates
If you want to keep a pleco and convict cichlids with other fish, you need to take their personalities in mind. Both the pleco and convict cichlids tend to hang out in the lower reaches of their aquariums. Additionally, convicts tend to consider the bottom of the tank their territory. So medium to large robust fish who prefer the middle and upper level of aquariums can make good tank mates.