Pure water - i think the ones we use have a 6mm tail on them. Would be interested in seal kits for central delivery ones too as was recently quoted £9 per seal as we have a few on our media panels with a crusty tip - oo err!
We normally use Walther couplings SP-006 (bullet tip on male) or LP-006 (flat tip) Not cheap but very robust and Walther are very helpful to work out what part number you need.
We use LV-004 for central acid - stainless not plastic as the plastic ones break easily.
Seals are available from Walther, if I recall housing seals were about £2.50 each and end plug seals were under £1.50.
We also have some Althoff fittings which are equivalent and compatible with the Walther fittings except the body threads are different, so hosetails are not interchangeable.
I got seals direct from Althoff in Germany as I couldn't find a UK supplier - I can't find what I paid. http://www.althoff.de/englisch/index.htm Chris