Welcome to our forum.
Someone here suggested the magic eraser. I tried it and it works awesome. That's what I'm using from now on.What does your hardness mesaure?
It sounds like your hardness is WAY TOO HIGH and who ever is testing it is mistaken.
I really don't mean to offend anyone, but has the "machine" been calibrated, ever? Are the test solutions they're using fresh? Have those that are operating it graduated from high school yet?Realistically the ONLY time you should need to adjust your hardness level is once, when you change the water. Evaporation will NOT lower hardness levels. Water lost to splashing and what's carried out on you will, but this should be quite minimal. It sounds suspicious that every time you get your water tested they tell you to add the exact same amount.
A gritty or sandpaper feel on the shell generally indicates calcium has precipitated out of the water onto the spa surfaces. If it is on the shell, it is also on the inside of the plumbing, pumps, etc. Calcium precipitation is usually caused by high pH. Chas has posted a fix for this in the past of lowering your pH to about 6.8 to draw the calcium back into the water because it is near impossible to remove it from the shell surface let alone the insides of the plumbing. Your sitiuation may be a little different because of adding calcium increaser, but the symptoms sure sound the same.