Thanks! Your tank ph makes sense because its pretty much the same as your tap. Looking at the pic, it looks like 8.2 but pics can be deceptive. Nothing to be concerned about!
Ok, I have no clue what they added to your water system to remove arsenic! Ill have to some looking to see if I can find out some more info. But it sounds like your water is being heavily processed. This is just something to consider- its possible that perhaps your water is lacking some key mineral if its being heavily processed. You can try adding a product such as RO Right or Seachems Replenish & see if this makes any difference. No guarentees but it also wont hurt anything either so its worth a try (sw section of lfs).
For now, wc & dose back to 2ppm. Keep us posted!