All of em shot with my Canon 300D with the kit lens (18-55mm). I use a +7 closeup filter (+1, +2, +4 stacked). Actually these were shot on full auto mode running with my Speedlite 550EX flash unit. In PS I adjusted the colors a bit to more accurately match reality (the flash tends to wash colors out a tiny bit when going through the water column). I also did a 3-step USM process to sharpen slightly. All shot from a tripod.
Beyond that, the really technically difficult part is shooting through the glass of the tank wall. This is through 5/8" thick tempered glass. Tough to control focus, flash-bounce, etc.