Fasting will help soften and break down hardened plaque. I have never water fasted long enough to have this happen, I tend to do many short water fasts. However last year I did a series of Master Cleanse fasts where I took no solids at all including the "P&B Shakes", then followed that cleanse with a Bowel Cleanse using psyllium and occasionally Bentonite (I cannot tolerate a lot of bentonite, it makes things worse) and had amazing results. I did this while eating a healthy mostly vegetarian diet. Remember that a lot of what comes out will be psyllium, after all it is a non-soluble fiber, but there will also be a lot of gunk softened, then scraped from the intestinal wall.
I would suggest that you not use water only fasting for this. It is too intensive. Try a 10-12 day juice fast or Master Cleanse, then do the bowel cleanse. If you want to try water fasting, then begin by doing a short one after your bowel cleanse, it should be much easier. The pre-fast preparation diet recommended here is very high fiber and I would imagine that it also gently scrapes the bowel and cleans it. However, toxins are not just in the colon, they are in the liver, blood, joints, everywhere. Water Fasting will help leach out this waste and allow for natural healing to occur. This does take time and long Water Fasts should not be done without supervision, this is especially true for novices and people with serious health issues.