I added 2 level tsp p/gallon Interpet Aqualibrium salt to my tank this afternoon (to treat finrot) and I only just now noticed on the salt packet the words 'With Extra Built-In PH Buffer!'

Thing is, my water is incredibly hard - the KH and GH are naturally very high already and the pH hovers btween 8 and 8.4..... so is this going to make the water too hard or raise the pH level even more?

Arg!!! Help!!!

Nope, Don't worry. Your Ph will probably stabilize at 8.4.

Jetman, you are my personal guide and guru today... thanks once again!!!

You might want to check your KH levels (+ GH, too, depending on the buffer they use) to make sure they aren't excessively high.

Thanks Happy, I forgot too mention that. Once you start getting over the 300ppm range with goldies you could cause some gill damage. Just use regular salt from now on and remember too replace the salt with any water changes.
Just a note but my Gh is just over 300ppm straight from the tap and I have never had any problems.