In order to sink, the sub has to acquire an average density greater than the surrounding water. The way real submarines do this is by taking on just enough water into their ballast tanks to make their average density of the sub equal to the density of water at the level they wish to descend to. To ascend, they force the water out by letting compressed air into the ballast tanks.
In order to make your sub descend, you will have to let water in - a small hole in the bottom of the sub should do the trick. You will then have to force that water out and replace it with air when the sub is at the bottom to make it ascend. A compressed air source in the sub that is regulated by a pressure valve (ie. it is open only when the water pressure is equal to the pressure at the tank bottom) would work. If the hole in the sub is small enough the air will gradually leak out until the water pressure makes the water flow in and the sub will start descending again and the process is repreated.