I don't know about the fresh water test kit, because i've never used one, but I would add water and salt to the tank, let it mix until clear, then put the rock in, just the way I wanted, then gently pour your sand (rinsed thoroughly if it's dry sand) in the tank, taking as much care as possible not to make the tank cloudy. This will happen anyway however, and it will take anywhere from 2 to 5 days for it to clear up.