Thank you chem geek. Your response raises other questions for me. As I have recently gained an understanding of how Ammonia works in pools, I've started to notice things in hot tubs that I believe is the result of ammonia. It seems to make sense that there would regularly be ammonia in hot tubs considering the relatively small volume of water and the amount of bather waste being mixed in. So what I am seeing regularly is that if I add chlorine, usually dichlor grannular, the TC increases, but the FC remains at zero. Exactly what I would expect for a spa with ammonia. If I add more, eventually I will start to see FC. I also use MPS in these spas, and for the last few years, my routine has been to add MPS, adjust pH, add chlorine, and check and adjust pH again. Now I'm wondering if by adding MPS first, I'm essentially wasting it. Is the MPS being used up by the ammonia?
I'm thinking it makes more sense to add chlorine first, then MPS, then adjust pH. The only reason I was adding the MPS first was because with test strips, the MPS shows up as TC, and if there is already FC present, its very hard to tell if you have added enough MPS.
I should mention that some of these spas use a Nature 2 system, and others use Bromine tabs in a floater.
I will say that since I realized I was seeing ammonia, many of the spas that have been very difficult to maintain have stayed clean and clear, so I think I'm headed in a good direction.