Your tank is very under filtered. The magnum provides no biological filtration and the bag of biomax in the AC is rated for a moderately stocked 70 gallon tank. If you've been rinsing the biomax in tap water then it is likely the gravel in your tank is what been keeping your oscars alive.
For now cut down on feeding and do alot of water chages to keep the ammonia levels lower. Also do a double dose of "Prime" water conditioner after each water change.
You need to add more biofiltration, there are many ways of doing this depending on your budget. You can also make your magnum into a biofilter if you still have the media container for it.
Once you filtration has been upgraded and is cycled you can then remove the gravel from your tank.
For now cut down on feeding and do alot of water chages to keep the ammonia levels lower. Also do a double dose of "Prime" water conditioner after each water change.
You need to add more biofiltration, there are many ways of doing this depending on your budget. You can also make your magnum into a biofilter if you still have the media container for it.
Once you filtration has been upgraded and is cycled you can then remove the gravel from your tank.
