I've only ever used tap water in any water-cooled engine - along with
anti-freeze of course!  Most modern anti-freeze mixes include things to
inhibit corrosion and clogging.

If the radiator is 'running low' then the system probably hasn't been
drained for a long time, and the hoses and gaskets may be in need of
replacement too.  Whether you do this yourself or get a garage to do it
for you, make sure that you know what anti-freeze has been added, so that
you can use the same stuff next time you need to top up.

If the pH is 10 then it is not "pure water"
<http://ga.water.usgs.gov/edu/phdiagram.html> - it's somewhere between
indigestion tablets and floor cleaner, and more than a little corrosive.

What Whiskers said. A mixture of tap and antifreeze. Check the colour of the
antifreeze, there are a couple of different types around now which aren't
mixable. Someone more knowledgeable than me will be able to tell you the
difference.

IIRC distilled water is what they call aggressive in that because it is so
pure, it can act as a solvent and start leaching metal away. This happened
in a linear accelerator in a hospital and so the manufacturer recommend
additives to prevent this. I would also question the quality of your still.
pH10 is highly alkaline as I'm sure you know, it should be pH7.