You can put this solution on YOUR website! Not a simple question.
You need to know what shape the 1000 liter tank has in order to forward a guess.
A cube would give a different result than a sphere than would a cylinder.
Imagine a cylinder that has a 10cm diameter and the appropriate length to make the volume equal to 1000 liters.
Here the balls would just rest directly on top of each other and not nest in between as they would for a larger diameter cylindrical tank.
Look up "sphere packing" and you'll see a whole host of approximate solutions but no exact solution yet.
