I'd seriously avoid the advice of that particular fish store in the future. When looking to add more livestock your best is to do it fish by fish unless there's an extenuating circumstance (like trying to circumvent aggression).
Adding five fish (all territorial) to a 20gal tank would be enough to seriously stress all the fish. Personally, my guess would be the boxfish did nuke the others. With certain species of boxfish it's a well documented phenomena. Of course, in a closed system the toxin should have probably killed the boxfish too. But the coincidence just seems to strong for me. Darren, you may've had a species that is not known to release the toxin or the toxin isn't as lethal such as Lactoria fornasini.
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lusus naturae
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