We frequently hear of corys that die in the shipping bag on the way home from the store, or shortly after arrival. I and many others have suggested that this could be ammonia poisoning, since corys are highly sensitive to ammonia and all other toxins. But in an article in the current issue of Amazonas
magazine, the UK author and aquarist Ian Fuller, an acknowledged authority on the Corydoradinae, offers what is likely the actual cause.
All species in the genus exude a white milky substance from below the gill area, possibly connected to the pectoral spine. This substance is extremely toxic to all fish, and kills within minutes. The release of the substance is triggered by severe stress, such as often occurs during netting and capture. Mr. Fuller includes a photograph in the article of this substance on a C. sterbai, and he notes that it is the only existing record as far as he is aware. In the confines of a bag of water, the substance kills the cory quickly. Small bubbles will appear at the water surface, and if the cory is taken out of the bag of water immediately and placed in fresh water, it may recover.
Mr. Fuller presents the view that this is likely a defense strategy on the part of the cory; if the fish is caught be another fish or bird, the automatic release of this toxin could result in the predator releasing the cory. The release of the substance is probably triggered by stress, and not controlled; thus, the cory releases it in the bag, even though its own imminent death will follow.
Mr. Fuller suggests that the frantic chasing of the cory around the tank during capture might cause the fish to release the toxin in the tank, rather than the bag. He also recommends taking home a second bag of the tank water, taken out prior to any capture attempts, in case the fish needs to be transferred en route. Once the toxin is released--and unless one actually sees the release when it occurs this will be un-noticed--the only remedy to save the fish is an immediate 100% water change of the container.
P.S. Yet another reason for those regular partial water changes; nothing else can handle this.