I DO however do all of my CLEANING with tap water - I've noticed that a lot of my SS tubing gets crystalline deposits on it when I soak it in no-rinse oxidizer, and I try to shake it off, but maybe there's enough lingering CaCO3 to have an effect?
For your other idea, is that a rest at 130 f BEFORE mashing at 150-155 or after? If I'm doing a 90-minute mash would I include the time used for the protein rest in the time for the overall mash?
I am afraid if CaCO3 is an issue in your house, the whole house filter will make the water taste better, but not do anything for the residue (unless by filter you are going to go Reverse Osmosis). Instead, you will need a water softner to remove some of the CaCO3... however, the salt it adds to the water will make it less suitable for brewing.
I have vienna like water with hardness around 275 to 300... so I know what you mean! The good news is that the CaCO3 is not a factor in your clairity at all.
As for the protein rest... it is derrived from decotion mashing used for undermodified malts. By resting at between 122F and 131F, enzymes cause the proteins to change and also, cuase the release of more soluable starch for the other rest steps to work. The common tale is that these days, with todays highly modified malts... that a protein rest is not needed... and for many malts that is true. Although, now that I think about it, I do a protein rest for my vienna/octoberfest beers. Those beers have a large proportion of Munich and Vienna malt that may benefit from the protein rest. Most beers that are mostly 2-row or pilsner malt, I do not do a protein rest. My useing a protein rest for Vienna/Octoberfest is just a hold over from recipies I used as a starting point. Since it produces awards, I have not strayed from the schedule on those beers.
Regardless, the 2 step decoction method starts with a dough in around 122-130 for a protein rest, pull a portion of the thick part of the mash to boil and add back int to hit 149-158, then rest until conversion is complete. Then pull the thin part of the mash(liquid) and heat to boil and then dump that back in to reach mash out temps.
Note, that starch conversion is going on at 130F. Because of that, you may want to step to a higher second temperature than you would normally do. Essentially, at 130F, you are getting some Alpha action, but no Beta. By giving the Alpha a head start, you may change the balance of sugars in your wort to be more fermentable and wind up with a thinner beer. So if you normally do a single infusion of 152, then you may want to do a mash schedule of 130F for 10 to 15 mintues, then ramp/infuse/decoct up to 158F until conversion is complete.
So after all that, what I think you may want to try is doughing in to 130F for 15 mintues, then stepping up to 155-158 F for 45 to 60 minutes. Do an iodine test to see if conversion is complete, and then mash out.
Don't forget to check for Calcuim...as CaCO3 is not the same as just Ca....