Subject: Many cave dwelling fish swim upside down when they venture around the ceiling of their grotto. The higher part of the cavern is made out of solid rock and thus hosts a lot of invertebrate species that can easily attach themselves on the hard substrate, while the bottom consists mostly out of sand. For the denizen fish of caves and caverns it is probably easier to find food or prey on the overgrown ceiling while swimming upside-down. In this particular clip there is, besides the Lionfish (probably Pterois muricata), also a juvenile Soldierfish (probably Myripristis kuntee) that occasionally swims upside-down.
Technique: When filming under overhangs it is important to have adequate light. Video lights must be correctly placed to minimize dark areas in the footage. Too much light or lights placed too close to the subject might result in overexposed zones. These burn out spots are very difficult to correct in post-production.