Instead of buying an hydration pack tube cleaner, I tied two knots at the end of some cord, slipped the other end up through the tube, dipped the dangling double-knotted end in bleach and pulled it through. Cleans like a charm. If you need to repeat, cut the dirty knots and tie two more.
BTW: I've been thinking lately that the hydration system is good mainly for bringing larger amounts of water to camp to be used for cooking and washing instead of drinking while hiking. Since we had a base camp last time out, this worked out better than lugging a water bucket there and also left me free to use just my lumbar pack with bottles on hikes.