Sealant

First, the tank. I live in a town that has very hard water w/alot of magnesium in it. Over time, the toliet tank has become black with mineral deposits. I've tried CLR, and other household products on it to very little success. Will anything clean it?? Next the bathroom sink; because of the hard water and a combination of mouthwash and toothpaste, the pipes form this slimy black gunk that sticks to the PVC piping. I've taken the elbow off and cleaned as far as I can, but there is a clog in the pipe where it makes a 90 degree turn in the wall that my fingers cannot reach. It's not a full clog yet, only a slow one. How can I clear this? Thanks.

I have a yard trap behind my house for the main line ans sewer line. My house is 106 years old and the main line is made from 5' sections of clay tiles. I have had the drain snaked and the plumber told me that I must have a colapse in the pipe because he can only get out about 70' - 80'. Well according to the city sewer guys where I live that puts them at the main sewer line in which with a camera the city guys say there is no problem visible 3-4' in my line. I have another company who snaked from the house to the yard trap with no problem and from the yard trap to the main sewer with no problem. How do I get the yard trap cleared?