Wanted to hear what you think caused the deaths

[ChargerstoLA]

TLDR on the post. Kid left the hose running to top off his parent’s pond for 2 hours. The next day all the fish were dead. 
 

There was some interesting perspectives from different people.
Some saying it’s the chlorine. But others have said the sitting water and UV would help eliminate the chlorine. Also they said it would take longer than 1 day. 

Some said the temperature difference  causing shock or sudden drop in PH.

An interesting one was about sudden water dilution on an established pond causing osmotic issues  

What do you guys think?

 

I also like his comment about a big water tank on the outside. 

[Wrencher_Scott]

CHLORINE for sure. PH and temp won't do that. 

[nabokovfan87]

Chlorine, contamination, PH swing, essentially..... you had OTS, and you massively changed over 200% of the water and shocked the fish.  Probably had poor aeration or something, not sure what, but the massive swing in parameters caused shock and that caused issues.  The fish care may or may not have been there before this happened, I don't know, but the water was low for a reason.

[MrWestCoast]

For all we know his water might be treated with Chloramines. If that's the case, chloramines will not evaporate like chlorine and will kill your fish within hours.