What do you think caused the death of all of PakMan’s fish’s?

Question

Yesterday, PakMan, a famous entrepreneur in the village met the village head complaining the death of his fishes.PakMan breeds his fish by channeling water from the river into his fish pond. He’s a very simple pond, with net as barrier and the water inflow and outflow depending on the slow natural water current. Save for some light poles to enlighten the pond at night, no other equipment is used. Being a tidy person, PakMan keeps his pond well-maintained, with not a single algae patch to be seen. Unfortunately, with his fish pond located downstream the river and downstream of everything else before the water flows to the sea, his activity is the one most affected by upstream activities. The village head recalls on how furious PakMan was and that he repeatedly mentioned the paint factory for being responsible.

The paint factory mentioned by PakMan has been operational in the village for three years.The lab test results indicate that the water sample parameters are normal

Answer 1

Possibly a lack of dissolved oxygen if the water in the pond is stagnant, or (as is said in the question) the current is very slow moving.

Also, starvation might be a factor if the fish used the algae as a food source.