I have tried some public aquarium shooting. I have some success with under expose the ambient light by 2 to 3 stops. Using only the flash as the main light source. This way, I can stop down to F3 or F4 (shooting a 135l F2) and still have fast enough shutter speed to freeze motion. I had to adjust the flash exposure manually to something like 1/8 to 1/16 power. Depending on the distance between the flash and the subject. I would also try to point the flash at an angle too.
The is shot in NYC Aquarium. ISO 1600, 250s, F3.2.
If certain part of your tank is more brighter, you can definitely try to expose the shot normally without the use of the flash.