I suggest putting the pots in a container of water and testing if/how the pH changes over a few days. A few days at least, because the pot will give off air bubbles for a day or two, and the CO2 in the bubbles will affect pH while that's happening. I discovered that terracotta pots I bought at Michaels made the water very acidic, around 6, which is actually okay for a betta but was so different from the 7.6 water I use for water changes that I was concerned I might stress the fish from pH difference everytime I did a water change. They also turned the water orange, more so each passing day. My tank was only 3 gallons so it would be subject to faster pH changes than a larger tank. I tested terracotta pots I got from Home Depot, and they raised pH to 8.something. Other people find terracotta pots that don't affect pH at all. Whether they affect pH probably partly depends not only on the clay the pot is made from (they're not all created equal), but also on the buffering capacity of your particular water, which is why I suggest testing it out. I also suggest filling the hole in the pot with aquarium silicone (before you get the pot wet) so that fishy doesn't hurt himself trying to squeeze through the hole.
