Ok, I see a few problems in this situation, that can be fixed....
First of all you should know where your cichlids are from in the wild, to try and make the best environment for them... Oscars are from South America, in the Amazon river. Jewel cichlids are an African cichlid that live in rivers also.
Oscars need a variety in their food, get some good quality cichlid food from the LFS and that should be good. Same with the Jewels.
Water Quality...
Both fish need good water quality to thrive, this can be hard with Oscars especially because they are known to be rather... "messy" in their eating habits. That is why they also require a large tank, not only because of their large size, it also has to do with bioload. You dont need to use any chemicals in a tank, except for a good water conditioner (takes out chlorine, chloramine, etc) PH buffers and other chemicals should not be used. A stable PH is better then a PH that changes often.
The African Butterfly fish will for sure get attacked and eaten by either the Oscar or the Jewel sooner or later as will the Dragon Eels...(never heard of Dragon eels personally)
Oscars grow to a very large size, about 12 inches as an adult, that is why most people reccomend a 75gal+ for Oscars. Jewels get to around 6inches. For your tank, it is already over-stocked so therefore, no more fish for you!
I would take out the Jewel and all of the other fish, and keep the Oscar alone, but even then, a 55gal is not exactly perfect for the Oscar.
That was for the first initial post... Now in response to Kephren01...
Oscars can infact, be mixed with other fish from the same area of the world. But that is not a common occurance because of the sheer size that Oscars grow too, therefore they are usually kept alone in tanks, or in a large tank with 2 Oscars. South American and African NEVER mix, totally different water quality etc. Just should never be done. Since Oscars are such messy fish, water changes weekly or more if possible are a must. Other wise disease could result in bad water quality. A pleco in that tank will just add to the bioload, you dont NEED algae eaters. Nothing cleans a tank better then a little elbow grease
Yes Kephren, Africans do like a higher PH, but just as long as the PH is stable constantly, that should be ok. The higher PH is ideal though. Salt is not NEEDED in the tank, it is usually used for medicinal purposes.
I think I've covered everything...If I havnt let me know