I thought about KH and GH as well, but it seems like all the fish would be affected by the sudden change in TDS. Here are some interesting posts I found a few days ago about pH, GH, and KH.
http://www.oscarfish.com/article-hom...-ph-shock.html
http://www.oscarfish.com/article-hom...-ph-shock.html
http://mtfb.com/MTFBJUNE/MTFB2%20pag...bsarticle2.htm