[Agent13]

Yep.. What Jaysee said.

BTW... what was copper doing in your water? Medications? or is it coming from your water source?

 

[Agent13]

Snails aren't sensitive IME ...aside from medications and salt. Don't know if there is a test for copper.
How long did your mystery snails survive? I have only had mine die after putting them in my semi aggressive community tank. How big were they?
Quick question which is more important then anything else I asked....is your tank cycled fully?And what is your maintenance routine?

 

[Agent13]

ooooh OK. Makes more sense now. Don't listen to those stores..come here and ask us anything you need to know. I suspect high Ammonia and or nitrites. Do you have a liquid testing kit? If not get one. Your fish are suffering to I suspect but they aren't showing enough signs for you to notice yet. Is your cory darting to the surface to breath? 5% water change is not going to help anything. If you are still in early stages of the cycle I recommend large water changes and often. at least 2 a week and personally I would recommend 75% water changes till you are near in the end of your cycle. Then back off to 50-75% water changes once a week with your 20g. once you are sure you are closer to the end of your cycle add more peppered corys for sure.
Keep us updated and good luck