DOes that mean your test strip doesn't have an ammonia test on it? Some do, some don't, but since ammonia is the primary toxin in a fishtank (because the fish themselves produce it), it is essential. With nitrite, there is only one safe or ideal reading, and that is 0ppm - If your test strip is indicating anything other than 0ppm, it is not safe or ideal. With nitrate, there is a lot of discussion on what is safe and what isn't. And the issue with test strips is that they aren't wonderfully reliable.
How much water do you change at a time? THe issue is that water changes really ought to be done weekly, in order to keep nitrate at a sensible level (in my terms that's anywhere between 5ppm and 60ppm). The problem with monthly changes is twofold - if you don't change enough water, it allows nitrate to build up to dangerous levels. If you change too much water, the sudden change from relatively high nitrate to relatively low nitrate can shock the fish.
My guess is that nitrate levels have built up too high, but it's only a guess at the moment, because you haven't given actual levels of ammonia, nitrite and nitrate. I would strongly suggest that you invest in a liquid test kit, these are much more reliable than the paper strips. I recommend the Salifert or Nutrafin ranges. Many people use the API, it's the most popular one by far, but its nitrate test is also hit&miss, because of the need to ensure that a powder reagent in the liquid is fully dissolved. It very often gives false-zero results.
The other alternative, in the short term, is to get your LFS to test it for you, but make sure they tell you the actual numbers of the results, not just "fine" or "safe" or whatever.
HTH.
