First and major concern is calcium. Fill water is basically rainwater from a cistern, and initial test gives calcium of 50 ppm, which I gather is low. Problem is that in my location I do not have easy access to pool shop or chemicals, so while I know it needs calcium chloride I cannot easily get my hands on it. I have read that the low calcium level will make the water aggressive. Pool is fully tiled in 1" tiles, so there is a lot of grout.... How fast can damage happen? Do I need to be looking at flying in a sack of calcium chloride asap, or can it go as it is for a few weeks/months? What is the absolute minimum calcium ppm in my tiled pool to be able to sleep nights? There is some limestone locally - would a block in the deep end be sacrificial??

Also confused about total alkalinity/PH. Total alkalinity is currently 80 ppm, which suggests borderline OK (?) or possibly to add a little more alkali (baking soda), whereas the PH is 8.00, requiring acid....(?) There seems to be a conflict here....

Any help much appreciated. I am totally new to all this and flying solo with the help of Google and this forum - huge thanks to all who have responded to previous posts.

Scott