You don't need to use a pressure cooker to sterilize your medium.
To clean/sterilize my hydroton. I boil 3-4 gallons of RO water on top of the stove. I place my hydroton (about 2 gallons total volume) into a 5 gallon bucket, which I pore the boiling water into and place a lid on the bucket and let sit for many hours, some times 24 hours. The boiling water will sterilize the hydroton and help to redissolve any salt build up on the hydroton. No chemicals needed, NO bleach needed.
To clean/sterilize my hydroton. I boil 3-4 gallons of RO water on top of the stove. I place my hydroton (about 2 gallons total volume) into a 5 gallon bucket, which I pore the boiling water into and place a lid on the bucket and let sit for many hours, some times 24 hours. The boiling water will sterilize the hydroton and help to redissolve any salt build up on the hydroton. No chemicals needed, NO bleach needed.