clay / ceramic pots in pond

hello,

I have an indoor pond approximetly 5 months old...water chemistry is
perfect. Recently my shubunkins and koi starting acting strange (crowding
together, skiddish). A pond expert reccomended a medication for parasites
and water changes. Both the medicine and water changes worked
wonders...however it is now going on a week and I am beginning to think it
is more the water changes that are helping and not so much the medicine.
Long story short...are clay pots toxic in ponds i.e. could they be
leaching something bad into the water? Perhaps this is why the water
changes help so much?

Thanks,
Leslie