Do fish respond to GH or KH?

> From: Erik "Dr. Clueless" Olson <eriko at wrq_com>
> Date: Sun, 21 Jul 1996 18:09:06 -0700 (PDT)
> Was just visiting cichlid guy extrordinaire Dave Soares, who said
> basically that the species' ability to breed corresponds to the
> carbonate hardness, because the harder it is, the tougher the eggs are
> (and less easy for sperm to penetrate them).

I always thought that general hardness (calcium, magnesium) was what 
affected biological things like eggs and internal stuctures.  Are you 
sure that Mr. Soares is also a chemical guy extrordinaire?  This
sounds like the typical confusion over hardness types and effects. 
I can't quite see how carbonates would affect egg toughness unless
there is a pH related thing going on.