I have access to Cidex OPA. I have read here and there that it is a potential substitution for Excel or Metricide14, but havent actually read of anyone truely using it. Just that it should work in theory. The MSDS states the active ingredient to be "o-phthalaldehyde" I'm not a total whiz at chemistry, but I know glutaraldehyde has a few different variations correct? I'd love to just use some Cidex for free rather than paying $12 a bottle of excel. Any experience with it? Who has actually used Cidex OPA specifically and had appropriate results?