Most of the time it is quite difficult to get inside to actually clean it properly. They can use chemicals though. Best would be to call a service provider and ask them what will be included in the price.

I see more and more people switch to tankless design, seems to be a trend. Tankless + sediment + carbon block. The reason is this horrible smell, I have no idea what happened recently, but some areas are affected, there's a lot more chlorine than there was just few months ago.