That stuff is a pain to get to dissolve. Your best best is to add it to boiling water. Might take you a few tries but basically you need to add the powder, let the water cool, let it sit for about 24 hours and check to see if any fell out of solution (will be a white precipitate on the bottom of the bottle if it falls out of solution). Keep re-boiling and adding more till it falls out of solution. That will tell you how much the liter of water will hold and then you know for the next time you make a fresh bottle.
However, even if you can get it to fully dissolve, it may still drop out of solutions after a few days (weeks?) if you dont keep the water circulated.