I personally would not use them because who know what kind of things that clay contains or what chemicals they used to cure the pot. But have seen people use them. I would give them a thorough scrub and rinse with RO/DI water. Let them dry out completely then put them in the oven and bake them on its highest temp just to burn off any residual chemical that may potentially be leftover from when they were cured at whatever factory they came from. Make sure you put them in the oven when the oven is also room temp and only bump up the temp 100 degrees give it an hour let the pots warm up to temp, then go another 100 degrees give it an hour and so on until you reach the max temp of your oven give it an hour and shut the oven off. If you put a room temp pot into a 400+ degree oven the pot will explode you gotta let the pot gradually get hotter. Let them cool down to room temp. Then one more scrub and RO/DI rinse. I would say after that go ahead and throw them in the tank.
They will cycle and throw off ammonia and nitrites into water, how much is any body's guess but obviously the more water volume you have the less of an impact on the tank the cycling will make. So just be aware of that and ramp up WC's according until the cycling is done.
Get whatever size you like best to fill the spot you want to fill. I wouldn't get anything overly big or underly small.
What I said may be overkill but at least you'll have peace of mind knowing that everything you could have gotten off yourself will be off the pot.
