You can use automotive antifreeze in it. Just make sure that you mix a single batch prior to filling the tube so it all has the same density. (It is poisonous, so don't drink it or dump it out in the field. Animals like the sweet taste of ethylene glycol and have been known to drink it.)
Ethanol mixed in with the water will also reduce the freezing point and will be consumable after the fact.
(You might want to use medical grade tubing if you plan on drinking the contents...)
Salt or calcium chloride would work too. (Salt is a terrestrial plant poison, don't know about calcium chloride.)
Doug