ceramic or agrocrete... i think you are confusing the two.
From a well know plug makers FAQ page:
Do your ceramic coral mounts require any curing or special handling?
No, ceramic does not require any curing or special handling. Our ceramic coral mounts are fired in a kiln at nearly 2000 degrees and are completely inert.
How many Aragocrete Plugs/Discs can I add to my system at one time?
The question of what is safe to add to a system at one time is nearly impossible to answer as the answer depends on too many different factors. Water chemistry like pH and Alkalinity as well as water volume and even water movement will have an effect on how a given number of plugs and discs effect your system.
The reason for a limit is that any aragocrete product will "cure" in water for a very long time (see above), this curing process releases alot of calcium into the water that can have an effect on your water chemistry. Calcium is usually concidered a good thing to add but the rate that calcium is added is not easily determined and can spike enexpectedly, causing potential problems. Our testing has shown that the bulk of the curing process is complete in the first 30 days so we cure everything in water for at least 30 days before shipping.
We have done some testing and have determined that one plug or disc per gallon of water per week should be safe to add to any healthy working system. We also always recommend that you watch your water chemistry during that week and be prepared for a water change or two to correct any issues that might occur.
Also, we always recommend that you continue the curing process by storing your new plugs/discs in a bucket of fresh water, change the water few days, and pull from the bucket as you need to use your plugs and discs.