I know this is an older post but just an FYI for this topic.
Prime does indeed remove/neutralizer the active component of bleach (sodium hypochlorite) but it does nothing to the stabilizing agent for bleach which is sodium hydroxide or lye. A typical plain bleach like HDX you can find at home depot is around 7-8% sodium hypochlorite and less than 1% sodium hydroxide. The chlorine part of bleach, the sodium hypochlorite is remove using prime as follows: 1 mL of 7.5% bleach (sodium hypochlorite) needs about 2.5 mL to 5 mL of Seachem Prime (a standard to double-dose) to ensure all chlorine is neutralized.
When your bleaching rocks to sterilize them, it is best to obviously rinse the rocks in clean rodi a few times toughly and then let them sit in clear rodi with a pump running to circulate water for a few days after as well, your can then use the Prime as an additional saftey measure. However if you throughly rinse the rocks a few times before then just letting them sit in clean well cirulated rodi for 48 hours afterwards, you should have given the sodium hypochlorite time enough to break down and be safe - they key is patience = saftey when dealing with bleach. If exposed to air and well circulated a 1 gallon bucket of water treated with 1ml of 8% bleach with be clear of the sodium hypochlorite in roughly 24 hours so 48 hours is just added saftey....
So when you use a highly concentrated amount like a 1/2 cup per gallon - after a total water change and rinse, your not dealing with super high concentrations anymore. At which point you can hit it with the Prime and let it sit a bit (few hours at least) and your good to go.
An added measure of safety is to use free chlorine test strip prior to using the rock. If the rock is in a bucket with a pump for an hour and you then test and it reads zero then your pretty well good to go.
