If your swimming pool looks like the birthplace of the Creature from the Black Lagoon, or if you just can't seem to keep the algae away for more than a day or two, you may need to drain and clean your pool. This is definitely a last-ditch procedure, and you shouldn't try it on your own unless you have the knowledge and skill to work safely with acid. That said, draining your pool and cleaning it can get your pool back to looking its best.
Turn off your pool's pump and filtration system at the circuit breaker. If you have an automatic fill valve, shut it off too.
Find the outside clean-out valve, usually located on the ground. The cap should be about three to four inches in diameter and should have a top that can be turned by a square wrench. Open the clean-out and place one end of a garden hose inside, ensuring that the hose is secure.
Attach the other end of the garden hose to a submersible pump. Immerse the pump in the deep end of the swimming pool, then plug the pump in. Go back to the clean-out to make sure the water isn't backing up. The Southern Nevada Water Authority recommends a maximum flow rate of 12 gallons per minute. If the water is backing up, you'll need to call a plumber or your local pool store to help. If the water is draining properly, wait for the pool to finish draining, but check the clean-out and low-lying drains in your home at least once an hour to be sure nothing has gone wrong.
Clean all algae blooms and stains from the sides and bottom of the pool using a pool brush. Scoop out any water or debris that remains in the pool.
Attach a garden hose (not the one you used to drain the pool) to a water supply and turn on the water. Leave the hose running at all times, and don't use a nozzle. Bring the hose, the muriatic acid and a nonreactive watering can into the pool with you.
Put one gallon of water into the watering can, then add one gallon of acid (always in this order, never in reverse). Starting with the deep end, spray a 10-foot wall section with the hose, then sprinkle the acid/water mixture along the section. Let the mixture sit for no more than about 30 seconds, scrub the section with a nonreactive brush for a few seconds, then rinse the section thoroughly with the hose. Repeat this step all the way around the pool wall.
Exit the pool. Neutralize the acidic mixture at the bottom of the pool by adding two pounds of soda ash for every gallon of acid you've used. Stir the liquid in the pool with a brush on a pole to ensure that all of the acid has been neutralized. Drain the liquid into the sewer with the submersible pump, as in Step 3.