I'm the original poster and just wanted to send a thank you to you all - the response has been amazing. Taking on board all your views I'll be using nice cheap deionised water from Halfords and PT Nuke clear (the non copper sulphate version with the name beginning with "B" I think). My full cover graphics card blocks are from EK and are nickel plated. EK put out a warning, I understand, about using silver killcoils with these in case of galvanic corrosion. I'm using the clear PT Nuke as, if I understand correctly, the blue is more suited to dyed water which I definitely don't want to use.
Thanks again all