Thank you all for your responses and reassurances! I'm on board with the reasoning behind the chlorine level required. I'm a logical person (civil engineer), and I value empirical evidence and calculations. I just need to adjust my 'gut feeling' to account for updated information . I really don't have a phobia - just a faulty basis, which has now been corrected.

So, from what BigDave said, and further discussion in the china shop, I have reluctantly concluded that my Nature 2 must go. I only say reluctantly because it's hard to see the $75 spent on the newly installed cartridge be worthless, but it can't be helped. I've never had my metals or calcium levels measured because I didn't think that it was important for fiberglass pools, but now I'm curious. I agree that it would be better to know what is influencing the pool chemistry rather having a black box that could be doing more harm than good, especially if it masks unsafe water by preventing the indicator (algae). At the very least, it doesn't change the FC requirement.

All that being said, I still need help with getting and keeping the FC level required. The pool is in full sun most of the day (96 degrees today and will be over 100 most of the summer) and the water was 86 degrees at the surface today. I usually add water every-other day to combat evaporation.

I measured FC at 0ppm last night, so I added 2 gallons of bleach at 9pm (18,000 gal pool). Today at 4pm I measured only 1.5ppm TC and FC. (TA 110 ppm, PH 7.5)

I'm guessing, based on the sun and heat and my current rate of consumption of FC, that I shouldn't try to lower the CYA of 80-90 ppm. Do you agree?

Then, if my goal is to keep the FC between 5-10 ppm for my CYA level, then what is the best way to do that? I was disappointed to see that the level was already so low by today, but I'm hoping that once I get it up where it needs to be that it will be easier to maintain. Did I not add enough last night or will I end up having to add that much every night?

Any guidance would be appreciated. Thank you in advance! I have a lot to learn, but I'm enjoying the chemistry so far

Ps- I'm using the Walmart hth 6-way test and also a Taylor drop kit (made for Leslie's pools - has R-0001, R-0002, R-0003 for FC/TC, R-0014 for PH, and R-0007, R-0008, and R-0009 for TA)