Toby,
Thats true, and i don`t feel comfortable with this low KH readings at all. Still i have a lot of limestone in my canisters, and also some in the main tank - and i have not experienced/registered any big pH fluctuations for a long time (i measure pH almost every day + some times before and after waterchanges).
My tapwater is pretty soft, and the pH some times goes down to about 7 - 7,1 after a waterchange, then some times it goes up again after 3-4 days to about 7,2 (depending on how much i feed of course). So there must be some kind of buffering effect in there... i don`t quite understand, but could be that i have had bad luck with the last two testkits - that they have been out of date or something...
Yesterday i spent 2k on a power aggregate (in case of long term power failures), so it have to wait... - But i shure will consider to look more into this Aquatronica divice as soon as possible

It absolutely seems like a good investment.