tsk tsk... The first question that should have been asked in response is the commonsense.. filtration system, gravel/depth of it, heating, ect ect... "What is your normal routine, and what type of equipment do you use? " It is true oscars are very big and messy fish. In a perfect world everyone would do 50% wc's weekly at least. haveing many tanks as alot of other mfkers do sometimes you have a week it just doesn't happen.
Live plants, overfiltration, understocking, food.
I will agree that if you are getting cloudy water regularly their is another issue to your problem then the fact you have "dirty" fish. You may simply need to change some of your equipment, or simply your overfeeding, or feeding bad foods. 2 oscars and a JD ime is also a recipe for disaster.
The fish mentioned I will agree are all "less dirty" But if it's the that is the issue, or feeding regime. You will run into the same problem again. The water changes seem to be the tip of the iceberg persae to me. and a symptom of something else that is your "problem"
The people here are very knowledgeable and helpful. and at this point imo no one has your answer.
More info is needed here.