I'm afraid Ethylene Glycol is out due the risk of poisoning the dogs if I get a leak and they lick it up. I'm tempted but my wife would never forgive me! The Proylene Glycol version is OK so I'm going to head for our local Auto-Pro and read the labels very carefully.

I obviously don't need the anti-freeze/anti-boiling properties of the stuff but some corosion inhibiters might be nice. I am also concerned about algae growing in the water. It doesn't get hot enough to kill the stuff but the ambient temperture here is perfect for it. If I don't find a suitable anti-freeze I might just go for adding a little chlorine based bleach (NOT the 'oxygen' stuff). A little further reading on Wikipedia sugests that a weak solution of 200ppm Sodium Hypochlorite will be strong enough for bug killing but won't attack the metals. Standard household bleach is about 5% (50,000ppm) so a couple of tablespoons of bleach in 10l of distilled water should do the trick.

Thanks for all the comments from everyone so far, this has prompted some interesting reading.

Kit