Some would recommend a water change if the ammonia gets especially high. A good measurement that I use would be if it gets over 1.0ppm. In reality, you don't *have* to do a water change. For my 40, while I was curing the live rock, the ammonia went off the charts, but I still had corralline, clams, worms, pods, etc. on the rock after it had finished. I would probably have more if I didn't let the ammonia go that much. It really just depends on the level of effort that you are willing to give.