If there's an official story, it would been out. From the pictures, someone had a bad shipment or a power failure, and decided to put them in the tank, took some pictures, and whipped up some lame story
Actually it was uploaded on SPCA's official facebook album. In case you didn't know, SPCA is the Society for the Prevention of Cruelty to Animals, which is a registered animal welfare charity with IPC status. Quite famous here in Singapore, the first thing which comes to most singaporean's mind when mention about pet abuse is SPCA. So I doubt this is a hoax made by some whacko seeking attention.
Here is their website, http://www.spca.org.sg/
Who in the right mind would put 29 arowana in a tank with an angel fish
As much as I hate to say it, singaporeans do... well for those who are ignorant about fishkeeping that is. I'm sure that most people here who do not participate in online fish forums such as AF or MFK don't even know what cycling it. And also they tend to overstock their tank thinking that as long as the tank can hold a fish, its big enough...
A great example is my dad... No matter how much I try to reason, he never does a water change on his tank and only turn on the filter when the water looks dirty...

No, not all of us know exactly what chlorine can do to fish. Some of us just assume chlorine kill fish, but don't really know what and how chlorine damages a fish
Does it matter? We know that you add chlorine into the tank and fishes die. We don't know how chlorine kills fish, whether directly or indirectly. But either way, we are gonna end up with dead fishes.