Yeah, I'd say that there was a major stocking problem. Those discus are far too large for that small tank, particularly with large angel fish. I imagine that no one told you that at the store, though.
If you don't want to buy a test kit, you can take a water sample in to most fish stores and they'll run tests. Some places charge a dollar, but its worth it. When you do that, post your results and someone will discuss them with you.
For now, I definitely second doing a large water change, and hold off on buying new fish.