I KNOW you've already had the water change answer as I've seen the other thread, and that really is the best way. Why do you not want to do it?
You can get various chemicals, or things to put in your filter to help, but all in all, a good water change is what is needed.
Also I know you are using the 5-in-1 strips, so if you haven't already it's probably worth repeating the test, as the test strips tend to give very 'approximate' readings IME.