Ceramic flower pots consist of inorganic (not living) earthy materials that form into a clay. The clay goes into a heating chamber or kiln to harden the material to make a more durable product. Terracotta which literally means "baked earth," is made from natural clay which gives it a characteristic reddish-brown color. The color varies slightly depending on the clay used.
As long as you are purchasing natural pots (not painted, glazed, or otherwise "cured" for decorative purposes), you should not encounter a problem. The natural compounds within the clay absorb and neutralize nitrates naturally (hence "ceramic rings" are used regularly as filter media), and their porous nature allows for colonization of beneficial bacterias. With that though, algae can also take hold, so make sure you keep them clean (scrubbing in tank, or removing and rinsing, using a new (unused for household purposes) sponge and softly cleaning surface). Do not use soaps or other chemicals, only water. They can naturally leach calcium and sometimes Iron as well, so keep that in mind when dosing with these supplements and check often for the first 6 months+.
-Em