Re: Lowering KH

> What about Muriatic acid?  It'll lower KH and I've not seen any 
> harmful side-effect.

Muriatic acid is the commercial name for hydrochloric acid, HCl.  HCl
is a strong acid that will dissociate completely in water producing H+
and Cl- ions.  The H+ will combine with bicarbonate ions (HCO3-)
lowering the KH a bit and will add to the equilibrium values of H2CO3
(carbonic acid) and free CO2.  

However, the Cl- ions will have the same effect on your plants as
adding salt (NaCl).  

Not recommended.