A much less expensive way to do this is to use a few more LEDs, and drive them at a lower current, so the heatsink doesn't need to be "high tech". If the light fixture has water hoses running to it, raising and lowering it will be a much more difficult problem. If you want the light suspended above the top of the tank it would also be very ugly with the two hoses going to it. If the light is in a hood, gaining access to the tank for maintenance could be difficult, since just lifting the hood would be a problem. All in all, I would go for more LEDs at lower current.