Rays are sensitive creatures.
their journey to your tank starts with the stress of being caught from the river.....then held in a viviero, then taken to the wholesaler and held then packed in a box for about 24 hours as they are shipped to a US importer/wholesaler..this is all kinda hard on them, multiple sudden changes in water quality , maybe not fed..... getting cold......ammonia issues....
Most wholesalers here then move them along as fast as they can, knowing that rays are a higher dollar fish and have an elevated mortality rate over other fish and not wanting to be left with a doa.....the last change, from S.American water to US water is a time when rays most require care and is usually the time when they least get it.
All of this can lead to the silent death that Ross writes about....the dmage done in the beginning stages takes time to show up...the first 5-10 days are a critical time.
