OK, one last time and I will be quiet.
The only thing is ... the other tanks, the seahorse and the 55 are fine. They all had water changes less than a week ago.
This has nothing to do with your water or the water changes.
Evidence from your explanations point to one thing as follows:
The only thing that stays common no matter how justifiable it is, is the Sea Apple. The other tanks that are fine, did not have one in the tank. I hate to say it but I think you are being a little naive on the fact that the Sea Apple could have released a very small amount ot the toxins out before the snails died and then when you took it out, it could have released more wreaking havoc on the entire system.