Subject: A shoal of Soldierfish (Myripristis vittata) hangs around in the middle of the water column near the entrance of an underwater cavern system. These nocturnal feeders of large zooplankton are extremely noisy. The emitted sounds are of such a low frequency though that they are inaudible to the human ear.
Technique: Filming in the water column is always more difficult than on or near the bottom. Stability is the biggest issue. Film slightly upwards and try to be as neutrally buoyant as possible and avoid jerky movements. In post-production you can always stabilise your footage provided that the camera movements are not too erratic.