How do I safely remove mineral build up from the inside of a stainless steel boiler tank?

I don't have a lot of room in the tank to move around with the heating element and the float that shuts the water intake on and off. Once I get my hand in the tank my hand blocks all visibility. Years ago I had some kind of stuff I could boil in the water with the system open to loosen the mineral deposits but it never worked no matter how many times I used it. Perhaps there is some new product that works now. This mineral build up on the walls of the tank are stuck like cement. I have always had to just get what I could out and forget the rest but after years of use it builds up.