With your current stocking, as the fish get larger, you will have to do more than 50% water changes a week to maintain less than 20ppm nitrates. Uaru get HITH/LLE much easier than oscars in less than ideal water conditions. Adding more filtration won’t reduce the nitrates or keep the total dissolved solid levels no higher than 50ppm out of tap. Let the fish reach adulthood (2-3 years). I’m sure you will have to shuffle things around.