"The reading was two parts per million (2ppm) which is very low," said a Fisheries official who declined to be named.
The dissolved oxygen of a Class 1 river is 7 parts per million (7ppm); Class 2: 5-7ppm; Class 3: 3-5ppm; Class 2: below 3ppm and Class 5: below 1ppm.
Effectively, the water quality of the lake just prior to and during the mass fish death was equivalent to that of a very polluted Class 2 river, from the perspective of dissolved oxygen content!
Fisheries officials reiterated the most plausible cause "may be" toxic chemicals discharged to the lake via the Complex's own drains.
"Many of the dead fish were found near the receiving end of these waste water discharged from the Complex," they noted.
"Maybe the waste water discharged into the big lake over the last few days was very toxic," the Senior Officer said.
But the precise process that led to or what caused such drastic dip in its dissolved oxygen content is not certain.
Often, decomposition of excessive dead algae can use up and remove much oxygen from a water body if there is an algal bloom (eutrophication) but a licensed chemist here said even a relatively amount of a potent toxic chemical dumped into a pool may react with water, which can also oxidise oxygen away.
If the latter proved true, it could validate the initial suspicion of a top Fisheries Official that toxic chemicals may have been discharged into the lake from the sports complex itself.
The Complex' General Manager and Resident engineer are, coincidentally, both on leave. Acting General Manager, Terrence Pudin, said besides the Fisheries Department, he had also asked the Environmental Protection Department (EPD) to conduct tests to solve the "puzzle".
"We'll wait for their reports, take their advice on what remedies to take to get the lake back to shape," Pudin said.
"We don't rule out the possibility of someone throwing poisons," he added.
"We are very sad that it has killed so many beautiful big fish which grew up from the fish fries we released into the lake in 2004, although it surprised us many small fish have survived," Pudin said.
"We found that the majority of the dead fish are Green chromide (Etroplus suratensis), a native fish of India and not Sabah," noted a Senior Fisheries official.
"I would have expected a lot of tilapia (native of Nile river, Egypt) but I was surprised there were very few tilapia," he noted.
"Also, we saw only two Koi (Japanese carp) and a 3kg grass carp among the cargo of dead," he said.
"The surprise was a big 3kg Milkfish (popularly called bangus in the Philippines)," he added.
Bangus are a brackish water (saline) species but it showed up in a fresh water lake in this case.
Some tortoises were also fund dead.