I recently got a laser that is water cooled via a couple of clear plastic tubes (about .5 cm diameter). I have heard it can happen that algae or other undesirable inhabitants can get into such tubes and cause problems. I have also heard that wrapping the tubes in something dark to keep light out can help with this issue. However, I have not heard of a good material for wrapping the tubes with, and I am looking for suggestions.
Ideally I would like to be able to slide a sheath of some opaque material over the tubes. Something like heat shrink (unshrunk) might work. I have also seen braided nylon sheaths for covering cables, but they don't look very opaque. What's the best option?