It's not unusual to see fish in malls. The photo above is in the Dubai mall. Hosts thousands of aquatic creatures with 140 species and over 400 sharks and rays. This tank is 150 ft x 60 ft x 33 ft and holds 2.6 million gallons.
The story making headlines is the Mall in Bangkok Thailand. This mall has an accidental ecosystem hosting 1000's of fish. It is also a health hazard, but people like to go there to feed the fishes.
Abandoned 20 years ago, the mall was too tall and a few stories needed to be removed. I could imagine the rain and sun started to create a new environment. The mall flooded and algae probably started to grow. Mosquitos and other insects soon discovered the bug paradise. Someone decided to a add a few species of fish to control the insects.
Part of this mall is already closed off for being too dangerous. And the future may end up closing and tearing down the ecosystem. Personally I think Bangkok can have a great attraction if they add a couple fountains and fix up the danger. Below are some photos.