Can't really help you. The liter measurement is the volume of the sand (how much space it will take up in the tank). The kilogram measurement is a weight measurement - how much the sand in the bag/container weighs. What I don't know is how much a liter of sand weighs.
Oooops - this was well answered while I was posting. Yep - 1 liter of water should be 1 kg. The sand will be heavier than water though (or it wouldn't sink), but it's still a good rough measurement comparison.