I have collected several species of box and cows, and they can and will emit the toxin when stressed, and (at least in our collection buckets) it will kill everything else in the buckets with them. I found a simple solution, just keep them alone until I get home. Once in tanks I've had them die and never had a problem with other fish, but I would not be suprised to hear of other hobbyiests who had problems, things such as tank size, O2 levels, other water conditions, all of these could have an effect if the fish releases it's toxins.