Home Water Softener

Never had to deal with one until now, and the previous owner just gave the bare minimum instructions for it (no manual).

My question is how do you clean the reservoir when it starts to smell of mildew? It’s a large 35-40 gallon plastic barrel that I was instructed to dump the salt in to a certain level. it goes through about six 40 lb. bags in six weeks. - If that helps any . . .