I got this for my Zojirushi hot water dispenser (get one if you don't, you'll thank me later), they sell their own cleaner but with some research, I found out I could simply use citric acid (2 tbsp), in fact I think their own cleaner is just citric acid. I pour some of this with some warm water, fill the tank up and turn on the cleaning cycle. The entire thing is descaled. This would work well with a regular kettle boiler too.
I had brown lime build up in my toilet, I could not figure out what it was for the life of me. Got different brushes, tried the vinegar soak, vinegar and baking soda, nothing worked.
So I decided to try citric acid. Shut off the valve, flushed and held the flush, then emptied as much as I could with an old container I had lying around, I poured some citric acid, probably 1/2C but I just poured til it looked right, and I let it sit. I checked in every 30 minutes. I set a timer for 3 hours but I didn't even have to make it to 3 hours! Every 30 minutes, the stuff would just flake off!! I would sometimes get my brush and brush the bottom of the bowl (where the pipe is?) and try to get the citric acid water mix into the pipes. After 2.5 hours, everything was GONE. It was amazing!! No more disgusting unsightly toilet bowl. I will be doing this every now and then when I notice a build up but I got a new toilet brush too (ends up I guess I didn't really need it though..)
You could potentially just use the citric acid mix and leave it for a few hours and flush and not need to brush since it just comes right off. I would say, don't leave it all night because it is still acid.