Man I just researched this out the wazoo
Things common between your tanks that could be causing a problem are only
Clay Pots & the wood you found
I've heard of people using clay pots many times with absolutely no issues
Found this link that talks about lime in aquariums and what they do to water
Sometimes cheap clay contains a significant amount of lime that is slowly releasing into the water column and will cause your pH to rise which in turn blah blah blah
Here's the link in case you're interested:
http://www.zx8.co.uk/water_quality_faq.htm
Also the wood that you said you found...Only wood I can find that would cause these types of issues is Rockwood...and I doubt that's what you found and put in your tank
Other than that I'm stumped
Things common between your tanks that could be causing a problem are only
Clay Pots & the wood you found
I've heard of people using clay pots many times with absolutely no issues
Found this link that talks about lime in aquariums and what they do to water
Sometimes cheap clay contains a significant amount of lime that is slowly releasing into the water column and will cause your pH to rise which in turn blah blah blah
Here's the link in case you're interested:
http://www.zx8.co.uk/water_quality_faq.htm
Also the wood that you said you found...Only wood I can find that would cause these types of issues is Rockwood...and I doubt that's what you found and put in your tank
Other than that I'm stumped