Problem aquarium water conditions?

I have a large tank - 6.5 foot x 2 foot, its been up and running for 2 years and all they fish are very healthy (never had a death), they are mainly oddball fish - 2 eels, 2 oscars, 2 tinfoils a couple of plecos and birchirs, a couple of rainbows and some catfsh. I recently did a water test and the results basically say the fish should be dead but there is nothing wrong with them. The only test that was ok was the ammonia, but I believe that this may be due to the extremely low pH - around 5. They nitrates / nitrites are also high. My problem is how to try and sort this out without shocking the fish. If I do a large water change and the pH rises, I believe I may run the risk of an ammonia problem. Anyone any ideas?