I suspect that you have a case of either bad genetics or just a stunted fish altogether before you even bought it.
I know for sure, from raising several batches of african cichlid fry, form the same litter, some will grow at normal pace, some will grow very slowly, and then there are a rare few that will grow twice as fast as the others... all in the same tank. So this is just proof that some fish get better genes passed on.
as far as the suspicion of stunted growth. the rate at which your oscar grew indicates to me that your water parameters are not that detrimental and are not the cause of your problem (although I'd still agree that only 0ppm of ammonia are acceptable). Considering that you have other fish, that have experienced proper growth rates, it leads me to think that the dempsey was in a bad environment before you even purchased him.
like others have suggested, I'd also suggest to try to improve his diet. I have seen very good growth with hikari products. try feeding them 3 times a day if it falls into your schedule.