Well, I have to disagree. Lime may be added to the product in the maunfacturing process, but lime makes the water more alkaline, raises the pH. And who likes water with a high pH?............did I hear someone say African Cichlids? South Americans and SA Dwarfs may not take a liking to the higher pH. But, it shouldn't raise more than .1 or .2.

As for using used pots, I would be afraid that some of the ferts and pesticides used in the pot's previous life holding flowers, that some of the stuff may harm, or kill, my fish.

I use clay, terra cotta, flower pots in some of my tanks. The Juliis, Buffalo heads, telmatachromis, and other smaller shell and cave breeders love them. I have even gone as far as to apply a gravel that matches the substrate to make the "cave" look more natural.

 

If in doubt, soak them and see if they change the water chemistry. I would bet there are different companies that do things a little different than others and you might see different results from many of them.