wkussmaul,
1. Your chlorine results seem odd. Are you using the FAS-DPD powder from Taylor using the 0.2 method or the 0.5 method, if not are you using their liquid, or another product test kit from another company? Are you using, pucks, liquid chlorine, or a saltwater system?
2. The CYA at 105 is too high it should be no higher than 80 (60 to 80 is recommended), see the CYA/Chlorine chart here:
http://www.troublefreepool.com/chlorine-cya-chart-t2346.html
For the recommended CYA/Chlorine level for a salt water pool, you would need to drain the pool down by 25 ppm or 24% to get to 80 ppm, or 45 ppm or 43% to get to 60 ppm (See edit below for non-saltwater pool levels).
3. As the CYA is too high it is probable that you might be using pucks, it would be better to use liquid chlorine as this adds no CYA to the pool, or install a salt system if you can.
4. The chlorine level is too low for a CYA of 105 (Target FC for 100 (not 105) is 11.4, minimum is 7.3), the suggestion that you might have algae seems then valid, as Vitamin C tablets will not remove the stain it is possible that you have Black Algae, nascent Black Algae (less difficult to remove), or at a minimum the start of an algae problem. You will need to raise your chlorine to shock levels for Black Algae, or nascent Black Algae, and shock for 4 -7 days maintaining the high level chlorine (higher than 46.1 ppm for a CYA of 80 ppm, if CYA is 60 ppm then 35 ppm of FC) for the entire time, checking every 8 hours to maintain the level for 4 to 7 days, and yes you can have black algae and no green algae, my neighbour had this issue under similar test results to yours.
5. You will need to monitor the chlorine level every 8 hours and make sure you pH is ideally in the 7.2 area, as the chlorine works better the lower the pH and the colder the water. It is nearly impossible to know the pH unless you calculate it correctly using the "poolequations" Excel spreadsheet by ChemGeek here:
https://www.troublefreepool.com/~richardfalk/pool/PoolEquations.zip
Do not forget to enable/permit to allow for "Macros" when using the sheet. The easier method to check for pH would be to obtain an electronic pH meter this will then give you an accurate reading very easily. Once you start adding this much chlorine for the shock treatment the pH rises and can offset the cleansing effects of the chlorine, so it is important to know your pH.
6. Doing the Black Algae shock treatment is a real pain as you need to scrub every 8 hours at a minimum. You can test for Black Algae prior to doing the shock treatment as follows, find an area where these marks are appearing and scrub hard with a steel brush and put a puck on it for 8 hours removing it every two hours and scrub. If the mark goes away you have black algae. In another area put another puck but do not scrub, if that algae remains you have confirmed Black Algae, if it to goes away then you have regular algae, or nascent Black Algae, so not so bad and a lot easier to be rid of.
If after 4 to 7 days the Black Algae shock has not removed all the stains you can treat as mentioned with a puck. If there are too many spots as was the case with my neighbour we had to drain the pool. We had the pool drained by sunset, we took cotton auto shop towels soaked them in pure chlorine, scrubbed the spots and stuck the towels over them, then scubbed and reapplied the towels every two hours, during the night, this removed the many spots (over 30 of them). You should do this in darkness as the sun can dilute the effect of the chlorine. You have to scrub the Black Algae as it has a strong film on them that is not easily broken even by chlorine, it also grows into the plaster so it is important that you get rid of them completely or the will grow back.
7. As mentioned I also would get rid of the Nature 2 as it adds copper and I think silver both of which can stain as it did in my pool, "The Blue Stuff" actually removes the copper and possibly the silver so in effect you are "chasing your own tale". If you live in a windy area or your municipal water supply contains iron and/or copper I would continue to use "The Blue Stuff", as I have a salt pool I use the "Purple Stuff" and it works great no more re-occuring iron stains from the wind born dust with iron in it.
8. To know how much chlorine you have in the pool at these high levels, say 25 ppm, you will need to add 10 times the amount of water to dilute the test and if it reads 2.5 ppm you are at 25 ppm. To do this take 100 ml of pool water and to that add 900 ml of tap water (measure your tap water to see if it to contains any chlorine and offset by that amount when testing), then pour mixture into testing tube to the desired level and test.
Hope this helps and of course it may not be Black Algae, but if it is you better get rid of it quick as it is virulent and nasty stuff, really hard to get rid of (especially if it grows on the walls), and will damage your plaster. Even if it is not Black Algae, which is entirely possible and I hope is the case, it seems that it is algae, or possibly nascent Black Algae, it would therefore be best to shock it out until it is gone, say 4 days to be safe. Once done use "The Blue Stuff" as this will mitigate/offset the potential for metal stains although it will be quickly used up by the high chlorine. You should scrub the entire pool with a stainless steel brush at a minimum of twice a day to avoid the metal stains appearing on the surface this will also help break up the algae. Hand vacumming to waste once a day also helps sometimes if a lot of algae falls off.
Finally sorry about the somewhat poor itemized/chronological orderI will try to re-edit it later so it flows better but if you read it carefully I think you will get the general gist of it.
Later edit for a non-saltwater pool: For CYA target of 40, drain pool down by 62% then fill with fresh water, target FC at 4.6 ppm, shock at highest level of 23.8 FC or above.
WARNING: IF YOU DRAIN THIS MUCH MAKE SURE THAT YOUR POOL WILL NOT POP OUT OF THE GROUND. CHECK WITH YOUR POOL BUILDER THAT SUFFICIENT RELIEF HOLES WERE DRILLED AROUND POOL TO ALLOW THIS 62% DRAINAGE.
If not drain 25%, then refill, then drain, 25% again then refill, etc. until pool has a CYA of 40 ppm.
