Husby is convinced that the blue tablets or anything in the toilet tank harm hardware, so we don't use them anymore and especially since we're now on a septic system. We're lucky we don't have manganese (sp) in our well water like so many folks in our village do (makes for a lovely blackened tank.) The inside of our tank looks fairly pristine. However, we do have bowl rings...
Bleach is a no-go as it tends to kill the good organisms in the septic tank. Denture tabs did not work on the waterline rings. We've tried Coca-Cola with fair success. (Turn off water, flush to empty bowl, pour a liter in the bowl, wait a couple of hours.) I've also heard that fabric softener sheets work well and will try those next (same instructions as above, but do not flush the sheets into the system.) Either method requires a little bit of scrubbing, but makes the deposits come off easier.