This is a article I wrote on the subject.
Ian
Avoiding self poisoning
Many people often encounter problems when transporting Corydoras, the number one of these and to a degree the one that is to be expected is that of bag puncturing. If you think about it we are dealing with fish that are equipped with very sharp pointed pectoral and or dorsal fin spines. To help avoid this type of problem it is always best to use strong bags and double them up, which is something that most retailers do automatically However when bagging your own fish for transporting there are a couple of things that can be done to help alleviate the risk of your Corys puncturing their bag. The first of these is to tape the corners of the bag back with adhesive strips, effectively rounding off the base of the bag and removing the pointed corners. It is the corners of where Corys dart into as soon as they are put into a bag, usually getting themselves well and truly stuck when they lock their fins, usually puncturing the bag in the process.
An alternative to bags and definitely a hundred percent safer are small plastic buckets or food containers with good fitting lids. They are strong enough to resist the Corys sharp fin spines and make the ideal Cory carrier.
Possibly the biggest problem encountered and the hardest one to explain is where Corys effectively self-poison themselves. What appears to happen is when a Cory is stressed it releases a fluid from either the base of the pectoral fin spine or the gills, at this moment in time the exact place where the fluid is excreted is not clear and as far as I am aware it has only been observed once. Although several people witnessed the event at the time and fortunately it took place while the fish were being exhibited in a show tank at the Catfish Study Groups Open Show in 2004 and where something could be done immediately to save the fish. It was spotted by one of the stewards and given a complete water change, after which the fish in question, a female C. sterbai recovered fully. At the time it looked like the fluid was being excreted from the base of the gills, but the prime concern was to save the fish rather than determine the source of the fluid.
What we do know is that in the confines of a small plastic bag, or in this case a show tank, this fluid release can be fatal. It appears to affect some species more than others and is brought on by stress. The first sign that there is a problem will be a small amount of foaming on the surface of the water in the bag and the fish will need re-bagging in fresh water immediately.
One of the easiest ways to avoid the problem when bagging your own fish is to firstly take a jug of water from the tank the Corys are housed in, then take some more water and put it into a suitably large enough container to temporarily house the Corys you are intending catching. Catch the said Corys and put them into the container, they should be sufficiently stressed from being chassed and caught to release the toxic fluid. After a few minutes if the fish are going to release any toxins there will be signs of some foaming on the surface of the water in the container. Now is the time to re-catch the Corys putting them into a bag or transportation container with the clean toxin free water from the jug, making sure not to transfer any of the contaminated water.
When buying Corys from stores it may not be that easy to convince the assistant that there could be a problem, but if you are in any doubt or you can see any foam forming on the surface of the water in the bag, ask for the fish to be re-bagged in fresh water. I am sure most store keepers will oblige as having customers arriving home with dead fish would not be the type of publicity they would be very keen on.
