your tank is overstocked ... a single oscar needs about 75 gallons at maturity and your arowana is going to need 300 gallons. your 55 gallon tank is already too small for an Arowana, let alone with the rest of your fish. while it may appear that your fish are immature right now and therefor can fit in your tank ... you need to be thinking about their adult size.
fish maintained in tanks which are too small or in conditions which are too crowded, develop severe stress syndrome. these fish do not exhibit proper color, do not develop proper fin form, do not exhibit proper metabolic development, do not exhibit normal behaviour, do not develop proper musculature, do not develop properly functioning organ systems and as a result, slowly lose their inherant resistance to disease. this results in a significantly shortened lifespan and along the way, lots of diseases for which the poorly conditioned fish is a good target.
ich IS a parasite. if your oscar has ich which has been treated with meds unsuccessfully, try salt. understand though, that if the conditions which allowed the ich or other parasite to take hold are not fixed, no amount of medicatin will cure the problem. have you checked your ammonia, nitrite and nitrate levels? NH3 and NO2 should be ZERO and nitrate is best kept at 10 ppm or less.
in a tank so heavily stocked, you need to be changing 50 percent of the water on a weekly basis concurrent with gravel vacuums.