if the tank has been set up for 6 months and they just did a water change yesterday, we have to act on the assumption that the OP has a handle on basic care/maintenance, so I doubt ammonia is the issue.
I assume the clay pots were new and just plain clay, correct?
is their any type of film or oil on the surface or foaming, etc, any indication of contamination?
Considering the course of events I would act on the premise that something was on the items and poisoned the tank. use activated carbon and do a huge water change, as much as you feel safe doing, you know your fish and their condition better than anyone online.
one other possibility, did you use tap water or RO/DI water for your water change and if tap water has it changed in any manner recently?