Acid washing a pool is a procedure to apply a diluted acid mixture to pool plaster striping away dirty and stained plaster. Because the process removes a layer of the plaster it isn't something that should be done every year but rather two to three times during the normal life of the plaster which is typically about 15 years.
Pool is drained here and ready to be cleaned.
Note the staining on the walls...
Touching up some spots with a fine grit sand paper.