Unfortunately it sounds like it might be too late for him... However if he pulls through the best thing you can do for this fish is buy a VERY large tank, or find him a new home ASAP. Assuming this is a common pleco, it can easily get well over a foot and a half long. Putting him in the 29g is probably better than cramming him into a 2g tank. Give him some hiding spots and observe to see if the other fish picks on him. He might be missing a fin just do to the stress of being in a tank that was way too small and the move. Also, was this 29g tank cycled first? If not, you need to start doing large water changes and test your water for amonia and nitrite, they should both be 0
Also, here is some info. on the Jack Dempsey, You will still need a larger tank than a 29g for this guy, even if he is the only fish in the tank...
Temperment Sociability Min. Tank Availability Area
Aggressive Solitary 45 gallons Common Middle
The Jack Dempsey is an aggressive, pugnacious cichlid which originates from Central America. The fish was named after a former heavyweight boxing champion based solely on its demeanor. The Jack Dempsey is an extremely hardy fish that can provide enjoyment for any level of hobbyist, so long as the caretaker is willing to provide the fish with the basic essentials.
The color of the Jack Dempsey changes greatly depending upon age, mood, and stress levels. A healthy young fish should be a brownish-tan with a black spot on each side of its body, and one on each side of the base of the tail. Dark stripes line the dorsal side of the fish, while blue to greenish-yellow spots are scattered around their scales. Adults should range from brown to black base color, the spots and stripes becoming less visible if not gone, and the brightly colored spots coat the body. Stressed, weak, or sick fish will be lighter in color, and the spots less visible.
The Dempsey is definitely not for the community tank. While they may be kept with other fish of similar size, the Dempsey is best kept in a species tank. Their aggressiveness should never be underestimated, and increases largely during spawning.
The Dempsey tank should have a thick layer of gravel, 4-5", with rocks of various sizes and caves for hiding places. Due to the digging nature of this fish, any plants which you might have in the tank should be well rooted, and do not be surprised if they are torn up every once in a while. Pre-soaked clay flower pots make for wonderful hiding places, as well as a good place for them to lay their eggs.
This fish is not usually very picky about what it eats, although it may take some time for them to adjust to a different food they are changing from a consistent diet. It is best to vary their diet, alternating between pellets, guppies, shrimp, lettuce, earthworms, snails, and most other crustaceans. This will provide them with the nutrients they need to become healthy, colorful adults.
Because these fish are from the area of Central America, they should have mildly hard water, with a neutral to slightly acidic pH. Although water conditions are not as strict as with fish such as discus, they will be happier and healthier with the right water chemistry.
Jack Dempseys spawn in the usual Cichlid fashion, the female laying her eggs on carefully cleaned rocks. The adults are model parents and both guard and care for the young.
Sexing the Jack Dempsey becomes increasingly difficult the younger the fish is. The most obvious ways to tell are with the male having longer and more pointed dorsal and anal fins, a longer body, and less blue-green coloration on the gill plate. The body of the female is more squat than that of the male, and the dorsal and anal fins are shorter as well.