This was a case I encountered a week ago, The whole population died within the day. Nearly all the bodies floated to the surface. Those that did not were trapped in the structures scattered around the area. The bodies were decomposing some were even missing eyes or if they were there had become glazed like frosted glass. Mouths were agape freezing their expression at the time of death.
The environment was dark and slimey. The sand almost pitch black. And the algae seemed to have perished as well. The water attained the dark color of poison. It was not tea colored like that of the Amazon blue or even green. No it was poison darkness if there was a name for it. Imagine smoke from a car’s exhaust combining and becoming one with water. This was what the water had become.
Even without doing the necessary check. One could imagine the ammonium, nitrate and nitrate levels of this aquatic environment. It was beyond the level any organism could live in. The organisms – fish and invertebrates would leave if they could.
But they cannot even if they wanted to.
This was a closed-ecosystem. A closed aquatic microcosm. An aquarium.
The filtration system that converted the waste substances into less toxic form broke down. The bacteria responsible for this action collapsed. And then the pollution level increased. Dissolved Oxygen was rapidly being used up.
Most likely the failure to change water contributed to this.
At the same time the automatic feeder kept on placing fish food to the system. Increasing the amount of waste from the digestion of the food and uneaten food.
The water became toxic and the fishes suffered and expired.
This was an ecological disaster. The Ecology could not cope with the population it supported and most important the microsystem was critically mismanaged.