A pleco in a jar will soon be a dead pleco. The pleco is feeding on a sick/injured fish, which is their instinct... they are closely related to catfish, and that makes them a scavenger.
Epsom salts will help to treat the swim bladder problem, if you can get the upside down fish to eat a few pieces of epsom salt, that should resolve the problem fairly quickly. BP's are prone to swim bladder problems, especially when not kept in a proper environment. If the cause of the swim bladder problem turns out to be a bacterial infection (which is likely) the salts will only resolve your problem temporarily. Medication would then be needed, and the most important thing will be clean tank and proper environment. If the root of the problem is not dealt with, swim bladder disease will result in permanent damage and ultimately death for the fish.
The plecos will feed on fish when enough other food source is not available or when they are feeling crowded.
Your photos show Gibiceps plecos, aka sail fin pleco, or butterfly pleco. They average about 16 - 18 inches full grown and they are primarily an algae eater, but will feed on vulnerable animals when available. Algae wafers, zuchinni squash, and special pleco and bottom feeder pellet foods are their primary diet along with live plants. These plecos are known for their huge appetites, and 2 of them in a 30 gallon tank is going to make finding enough food very difficult and eventually impossible. Taking the rock with algae out of the tank only took away their food supply.
Gibiceps are also territorial, and need places of their own to hide.
After reading your most recent posts and seeing your photos, I have to say, you don't have an appropriate environment for these animals. They need a much larger aquarium, much more decoration... in your tank's current condition the problems you are having don't surprise me, and you can expect these problems to continue and get worse as time goes on. You really only have 2 choices...
1. Move the fish to a suitably sized tank, or
2. Watch them die
Unfortunately, there isn't anything more you can do except that. If your father is having an issue with providing proper care for these fish, then it would be best to get rid of them and give them to someone who can. Putting a pleco into a jar is not going to resolve your problem, and the other pleco will likely canibalize the fish as well... and soon. I know you've said you don't want to hear about the overcrowding issues you have, which are quite obvious from your photos, but the overcrowding is causing the problems.
I know some of the other members have grown frustrated here, and that is understandable. I, too, am growing frustrated with this situation. I don't know what more we can do to help you, and dealing with each problem that arises from the same cause, while knowing the cause is not being treated, is frustrating to all. Each time you post with another issue in this tank, the answer is going to be about the same... they need a much larger tank or they will eventually die from these other problems. You cant treat these other problems enough to get rid of them while these fish remain in a 30 gallon tank together, it's not possible.
If these animals are inhumanely treated, like putting a pleco in a jar... then I will not come back to help here in this thread or any others you post about these fish, and I am sure the others are going to echo my words on that. This whole thread tells me that you have acted very irresponsibly with these animals and are continuing to do so without regard to the animal's safety. I will not do anything to contribute to that.