clay pots in tank

You can rinse them off in cold water or boil them for a few seconds and then rinse them with cold water. Just make sure that they are brand new, never been used before. Thats about it.
 
You can use a masonry bit in a drill... or, my personal fave, a hammer! (Then again, you might not be going for the "pottery shards" look, rather an actual "fishy house, so the drill method would probably work better for you...)

If you have a dremel tool, that's the best though, because you can cut a big hole in there in whatever shape you want... make a little fishy-Taj Mahal, even...
 
I made a dandy entrance hole in a clay pot with a dremel tool!
If you start out with an "orchid pot" (they have enlarged drainage holes on the sides of the pot) it is much easier to get started drilling it larger. I siliconed tiny broken pieces of clay pot over the other holes (on the inside of the pot) so there is only one way in, one way out.

cubbypot2.jpg
 
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