Seriously? We goingthere.... again... hahahaha.
@hadiosbourne
As Dallas stated, the product is sold per volume, not per weight.
To be more precise, the product is actually sold per volume needed for nitrate removal.
Don't have the exact details at hand, but if the bottle says 500ml will treat 500 L of water, then effectively, you are paying for nitrate treatment of 500L of water for every 500ml you buy. regardless whether said 500ml weighs a kilo, a gram or a ton, the quantity in a 500ml tub will treat 500 litres for arguments sake.
Another technical answer, the "salt" (apparently they taste salty, therefor they must be salt

) crystals in the cubes will dissolve once soaked in RO, and this will reduce the weight further....... again, irrelevant, as it will still treat 500 litres of water.