Totally agree, i bought 80 pounds of seachem aragonite, works out expensive, but 2nd best just doesnt cut it with marine care. Rather save up some cash and then invest in aragonite.
deep snad beds are usually 4 inches ore more averaging at 5 inches but can be up to 12 inches. a plenum is basically an empty area under the deep sand bed that gets little oxygen meant for better nitrfication