My experience of shooting in aquaria has taught me one thing - that the fishy bits are always significantly brighter than the peopley bits
In terms of portraiture, it's like shooting directly into a setting sun, with the added problem that there's glass everywhere to play havoc with your flash.
You can get good shots using ambient light if you shoot parallel (or at a narrow angle) to the glass and your subjects are close to the glass. You'll need fast glass for this. If you put your subject between you and the glass you're going to end up with silhouettes.
You'll have to use flash to expose the people properly, so you really have two options: 1. Be really careful with your angles to avoid reflected flash (which means no gorgeous shots of people with fishies in the background) or 2. get your flash(es) off camera.
You'll get the best results with a couple of off-camera flashes if that's available to you, using snoots or flags to keep the flash off the glass if possible.
You've obviously worked out that this is a significant challenge - in fact one of the most challenging lighting problems I can think of.
That said, it's a great learning opportunity (for everyone at DPS too). I'd love to see the results.
Good luck.