Dry rock.

Pool acid is usually muriatic acid. Be careful with that stuff. Special container, the acid, the base to neutralize, safety gear.... Cannot be left unattended, if you use too much it will cut through calcium based rock. It is also an attractive nuisance to children etc... I would just cure it in a bucket, it is dead. Microwaving rock for 20 seconds makes sure all spores are 100% dead if you want an added layer of assurance.

 

You can buy it at Lowe's or Home depot.

Here is a safety link should you go that route:

http://www.mass.gov/eea/agencies/massdep/recycle/hazardous/muriatic-hydrochloric-acid.html