You did everything you could though. Tankmates (or their death) can easily effect the well being of other fish, especially a delicate pups. It is something we all battle. Probably everyone out there who keeps rays has had a similar experience where things just went wrong. We certainly have.imma pick up some ammo-carb today put that in my HOT magnum Currently it just has Carbon in it.
I run both a Hot magnum on the Tank and the sump. with about 7 boxes of Fluval Bio-max rings