that might be your problem... To do a 75% water change is way to much, especially every week. Even a 50% water change is to much. Try more like a 25% water change. If you have a good biological filter of bacteria you shouldn't have to do so many changes. It is to much stress on the fish. Some fish are more resiliant then others, so that might be why only the oscars died. And depending on where you get your water, it may be to hard or to soft. Take a water sample and bring it to your local fish store and have them check it. Oscars like harder water, but tap water usually is softer depending where you are.