They definitely propel themselves. Even if I stop the filter so that there's no water circulation, they can walk on the surface just as fast as they do on the

They definitely propel themselves. Even if I stop the filter so that there's no water circulation, they can walk on the surface just as fast as they do on the glass (which is quite slow, but still). Think about it: water striders propel themselves quite fast on the surface of water, from the other side. The air/water interface is a though barrier to break for small beings. Thus they can use it for support, from either side.