How do I raise GH without affecting pH or KH?

[daveedka]

/Until you add enough to cause a problem with High TDS levels there should be no side effect. and with Hard water fish it would take huge amounts to do any damage. once a level in the tank is estabilished, then change water should be adjusted to match that level.
Dave

 

[happychem]

That's basically what I was getting at.

Anything can be "harmful" if you add too much too fast.

 

[RTR]

Any soluble material added to the tank raises the TDS and thus increases the osmolarity/osmotic pressure experienced by the fish. As said before by happychem and daveedka, too much change too fast is risky and can even be fatal. There is no such thing as "pH shock", but if you change that to "osmotic shock" it is quite real.