I would get a hose and mix up 10 parts water to 1 part bleach. Scrub down the walls with a brush, sponge, or scrubby pad. Once you have it nice and clean, rinse it well and let it air out for 24 hours to dry. It will be ready to use. If you do this annually, that should suffice.
If you want to use Muriatic Acid (HCL), that works faster but you'll have to figure out where to dump the acid solution when done. I clean my 265g poly tank with acid and water. Using a hose, I add about five gallons of water into the empty container, and a quart of acid next being careful not to breathe in. Then I lean the container over on its side, and slosh the solution back and forth over a 2' area. Then I turn the container a little and slosh the next area, and turn, and slosh, and turn and slosh. Once I've turned it 360°, I can stand it up again, and drain the solution out. A garden hose with a jet attachment allows me to rinse the walls well and flush out all of the solution and specks of shale remaining. It's like starting over with a brand new container. I do this once a year.
