Well, at the public aquaria it depends on the exhibits. Most tanks I can still do overhead wireless stuff like always, u just raise it up and point down, becomes almost like a straight down angle.
Jelly areas are usually pitchblack and it's impossible to use flash. Trust me I tried. Then it becomes a matter of a camera that is good at high ISO shots. Most Canon DSLRs are exceptional (one big advantage over Nikon) at taking high ISO Photographs. These were done at ISO 1600 and are still very usefull.