Oh, and at all expense do not use flash!
Davec223,
Nice shot you have there
I have to say, thank you for the tip on the flash. I've never been able to get it right, hence why I do not use it. I also think the OP may find it very hard to use one off camera while in London Aquarium. Still, a very good tipto have.
You shouldn't really use a flash around fish, they are very sensitive to changes in light and most do not have contractile pupils. The more hardy fish can take it but don't particularly like it. Sensitive animals like octopus or sea horses may die if they have a flash fired at them.
I'm not sure the bold bit is correct.
I'm an underwater photographer and shoot with a number of U/W togs who are all very respectful of the environment in which we shoot. I know all of us have used strobes when shooting seahorses and have never been advised this has the potential to kill them otherwise we wouldn't do it.
We don't repetitively shoot any animal to respect distress that could be caused by strobes, but I have never heard of seahorses and octopii dying as a result.