Update: they're all doing really good & eating well, and really happy like before the incident happened. I continue to used aged water for nightly WC and will make a determination on this next week.

RE: Zinc Orthophosphate in aquariums: http://www.willegal.net/tropical_fish/algae.htm

Couldn't find anything specific here in the archives, but found the above link on a google search. It basically says the Zinc Orthophosphate is added to prevent pipeline corrosion, but it can cause algae blooms and mentions 'red algae'. So it doesn't sounds like a harmful chemical to fish. And makes sense since during my tank wall wipedowns the film coating has always been reddish in color.