Water kills a fighting fish

I ask the aquarium shop why does the live prawns I brought back for my Arowana never survive overnight in the fish tank? Most died within 24 hours while the stronger ones survive the mysterious death for another day. That Aquarium shop staff told me that nowadays the prawn they got seems to be dying easily despite higher concentration of air bubble and use of leaf water.

A man step into the conversation and told me that its all the "N-Water" fault and he had some rather shocking evidence to prove it. His tale starts with how his good friend and avid Arowana owner did a regular water change, which had always been a peaceful occasion. However one day all his Arowana died after a water change. It sure is a tale to me for Arowana are champion survival even in poor water condition however I took his advise never to change more than half a tank of water each time and put in anti-chlorine and salt before adding in the tap water to avoid "shocking" the fish.

2nd tale involve a personnal experiment done by him. He got a bottle of "N-water" and pour it all into a small little tank where a healthy Fighting Fish lives in. Within 15 mins the colors of the fish faded to dark brown and within an hour the fish is dead in the "N-water". Astonishing tales to me!!! How can it be possible? Its clean and safe water by the way....

3rd tale. Young man collapsing while participating in running even. They use to distribute "N-water" at each drink station but stop after a while and nowadays its all Isotonic drinks. Hmmm...there is no prove to this claim but still surprise to see he was damn convince its the root cause.

I'll take the fact with a pinch of salt for they are just tales but I might wanna try the "Fighting Fish" experiment during the school break.