Worried About Cardinals

[star4]

Hi I did a water change yesterday on my son's tank. I also had to remove a lot of plants as for some reason they had all died off suddenly. While vacuuming the gravel I noticed lots and lots of empty trumpet snail shells, they have all died off suddenly too? The water stats are now fine, but some of the cardinals lost all their red colour last night? They seem ok this morning and have eaten and their colour has come back. Just a bit worried.

 

[WILDER]

Can you post your water stats please.
I would add some fresh black carbon to the filter as rotting snails can become toxic to fish.
Any signs of darting, or laboured breathing.