This is not totally true.
Low pH does mean that more of the ammonia is in the less toxic form but pH has no effect on nitrite or nitrate. However, less toxic doesn't mean non-toxic and the only 100% safe level for ammonia is zero.
What is your tap water nitrate level?
Nitrate in a tank should be under 20 ppm, but many areas have more than that in tap water. If yours is well under 20 ppm, your tank reading of 40 ppm is way too high.
Water changes are the way to get ammonia down, and also nitrate if your tap nitrate is low.
