Sooner or later the plastic tube on your hydration bladder will start looking less than hygienic as unspecified biological matter begins to grow inside. This can be very difficult to clean out but Milton anti-bacterial tablets which are used for cleaning babies bottles etc. works wonders. It is readily available in the baby section of your local supermarket and relatively inexpensive. Just add a couple of tablets to water and soak for 15 minutes. Stubborn cases may need overnight or repeat soakings. Give it a try.