Just stop adding chemicals and doing as much water changes as your doing. As suggested in prior posts your tank crashed for whatever reason and now needs to be re-cycle. By continuing to clean the filters, adding chems etc. you are not making it any better.
As long as your fish are OK, i'd let things be and let nature take it's course.
I had the same problems as you with my 150g. I kept trying to fix things and the more I tried the worse it got. I finally gave up and after a couple weeks everything fixed itself.
I think you just have to be patient rather than reactive with the ammonia.