Terra cotta does not usually affect the pH or hardness or any other parameter of the water that I have noted. The one thing you may want to do if you have smaller fish is to make sure the pots do not have sharp edges. Try rubbing them with a nylon stocking and where the stocking grabs take a small piece of sandpaper and rub the roughness off. Fish can be torn or fins broken with catching on these rough areas. Be sure to check the drainage hole in the bottom as well. If you have a betta plug the hole in the bottom as I know of many bettas who have gotten half way through the hole and gotten stuck. They are terminally curious and think they have to go through everything. If they get wedged tight they cannot go to the surface to breathe and will drown.

Rose