Using playground sand caused the tank to be very murky upon a first filling with water. I waited a while to see if the sediment would settle but it did not. So I emptied the tank and filled it with fresh water. After noticing that the water was substantially clearer, I emptied the water one final time and filled it up slowly, resulting in a clear tank, ready to shoot.
I knew I wanted the fire relic to glow, so I bored a tunnel in the sand below it and used one of my flashes solely to illuminate that piece.
After wrapping up the photo that night, I emptied the tank and broke down my set up. It felt good to be finished but I was not 100% pleased with the result: the helicopter in the background was a bit difficult to notice, the sunken relics were not in the best position, so the following night I set up the entire scene again and got everything to a point I was happy with.