I prophylactically treat all incoming fish, so it's hard for me to say which were diseased upon receipt and which were not. But usually with every shipment I can see active signs of velvet and/or uronema in a few of the bags. These fish are isolated away from my main QTs and almost always die. Too far gone I suppose.
I get in around 50-60 fish at a time, and perform necropsies on every single fish that dies. Most of the time I can trace the death back to velvet, uronema or a gram-negative infection. Sometimes only the liver & kidneys look damaged, which suggests either cyanide or ammonia poisoning. Some shipments I only lose a few fish; however this last shipment has been devastating because most of the fish developed a gram-negative infection that I initially mistook for Lymphocystis. White lesions are even spreading
internally over the fishes' internal organs. Guess I got really unlucky this time around and picked up a weird bacterial disease. Staining has confirmed it is being caused by gram-negative bacteria, but I am unable to ID the genus, species.
White stringy poop is a symptom of internal parasites/intestinal worms:
https://www.reef2reef.com/threads/internal-pathogens.267419/