from now on i stick to clam chowder

[Aquattro]

I added 1" maxima to my brand new 90 3+ years ago, and moved it several times. It now resides back in the 180 and is almost 3". So you can add to a new tank, moving around is fine if care is taken to not damage the foot, and I've never directly fed it.
While these things can affect them, it's more likely parasites or some disease. We often read about mass die offs every now and then from unknown causes.
Did you ever look for those pesky little snails?