Personally, i'd be surprised if many fish react badly to melafix, its a pretty harmless treatment with no chemicals in, if anything, the waterlife treatments can be far more harmful (potentially) to more sensitive fish!
I have been using full dose of melafix and pimafix (not at same time!) in my own tanks with discus, angels, clown loach, severums, cories and danios etc with no adverse affects.
I also run full doses through all 5 systems in the shop so every single fish in a particularly large aquatics shop, including marine (though marine version and lower dose slightly) with no adverse affects. If i ran a full dose of Myxazin or such like through a system i can guarantee i would wipe out a lot of fish, loaches, pencil fish, black ghost knife fish and so on would be wiped out.
That said...
In this case, my money would be that this is a combination of several things, namely a scrape on the side possibly leading to complications, maybe internal maybe not, most likely bacterial, shock from moving is always incredibly stressfull and will lower the immune system hugely.
Personally I would run a half dose of Pimafix and a half dose of a waterlife treatment for Bacterial Infections (myxazin i think?). API promote melafix and pimafix as being a herbal treatment to be used alongside other treatments to boost their effectiveness.
EDIT: Im not surprised there is bias if the member works as a consultant to Mars Fishcare
they make the Melafix and Pimafix! (Mars, not the member!) Why would you not want to promote your own products if its what you believe in?
The API or Mars products are actually very good and very reliable and i couldnt run my tanks without them! When i had my motoro rays, they were forever scraping themselves (in a bare tank! I have no idea how!!) and Melafix cured the cuts and grazes in a day or so!
There is no harm in promoting a very good product. Same as Stress Coat and Stress Zyme are very good, as is their PH down.