When you even encounter the sand gates with some sand guards, kill the one that have red color on his threads(the one that will send reinforcements if you have alerted the guards of the sand gate)and then take his sword, that sword has the ability to restore the sands on your sand tank without getting sands from other breakable objects around you. This sword will be gone if you thrown it or you have been transform to Dark Prince, but don't worry there are many sand gates that you'll encounter in the game.
After you save at "the fortres" save fountain you will see a yelow beam. It will give you a new sand tank. Now go to the left and do a wall run (there will be a trap on the wall). Jump from the wall to the pole. From that pole jump to another one and jump again. Now kill the enemy. Jump to a chain and go down. There will be a few enemies that you need to kill now. When you kill them go to the left. Now you can see a little item near the cross bar (bars). Break it. Now prees "c" to pick up weapon that was in that item (it looks like breaked piece of dors or window).