I used them all the time when I had fish (back in the old days)
If you start with new pots, you shouldn't have any problem. The owner of a garden centre told me years ago that it's illegal to sell clay pots with any kind of contamination, since they pot might be used for growing food plants. I don't know if that's still the case, but I don't see why not. Even if it's not illegal any more, it still makes sense to manufacture only safe ones. Unglazed ones don't even have the added worry of the glaze chemicals leaching out.
To play it safe though, I always soaked the pot in water for a few hours. Next I boiled it for about 5 minutes (if it didn't fit any saucepans I had, I soaked it in boiling water in the sink). A good drying in the sun should take care of any problems.
Used pots could contain traces of any insecticides and herbicides that were used on the plants, and even fertilizers so they could cause problems. New pots should be problem free. If you don't want to try them again though, I've had good luck using the plastic clay look-alikes, but they need to be weighed down.
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