I've killed fish photographing them with a flash in a photo tank...I felt just awful about it; I let the drive for me to get a good image come above what I knew was right for the fish.
How can you prove that you killed them with the flash? Did they up and die right then?
I've photographed literally thousands of fish with a flash, my trusty 580EXii and I've NEVER EVER seen anything even remotely resembling a life threatening reaction. The most I ever see is a fish darting away a little bit then coming back out. I took pictures for a fish store, of every single fish they had besides things like damsels and chromis, for their website, and I did that for over two years, and never saw anything like what you're describing.
Now, granted, very fragile fish like sea dragons, I can understand if you kept flash away at them and stressing them out, they might go off food... but did the flash actually KILL them or did you just notice them stop eating at the time of the photo session and never resume again?
I've never heard of a mantis yet that's been killed by a flash, I'm not sure how you say it is common. I browse three different fish forums and am a frequent visitor of the mantis page on all of them. I have a
G. smithii, commonly called a purple spot mantis. He certainly doesnt mind flashing, he comes out and eats literally 2 seconds after I get my pictures. I've never seen or heard of anyone claiming a flash can kill a mantis. I do know peacock mantis can be susceptible to long term high light exposure... but we are talking living in a reef tank, not a couple bright flashes...
Octopus I have no experience with taking pictures of in tanks, but in Hawaii we photographed quite a few. They were the small variety, under a foot with tentacles spread, but we all used flashes, some people had double strobes and major cameras set up (this was before I was interested much in photography) and I certainly never saw anything resembling stress, most of them just changed colors and looked irritated.
Anyway, I'd like to hear more on your story/experience. It just sounds like an old wives tale to me, to be honest. I'm not the worlds best photographer and I'm certainly not the world's most prolific photographer, but I've done thousands of fish photos (30,000 shutter actuations on my 50D and at least half were fish photos for the store) and I've got about 15,000 on my 5DmkII, probably 25-30% of them are fish photos. I'd say half of all the store photos were flash photography before I had full frame and could have a useful high ISO. All I'm saying is I'm an experienced fish photographer and never in my years have I seen what you're saying happens happen.