If your talking about clay pots, I would rather buy new ones since they are so cheap. I wouldn't use one of my wifes clay pots with all the fertilizers and crap she puts in her plants. The clay is a little porous too. A lot of soils come with fertilizers and pesticides in them also.
Go buy some new ones, they are so cheap it is not worth the risk of having contaminated ones in with your fish. Clay pots are very permeable and soak up everything that is dumped in them. If the pots are ceramic it should be ok with a good washing.
Yeah dude, better safe than sorry. Get some new ones. If your lucky you can find a place that has broken ones. They'll sometimes give them to you for free. That's how I get mine.