Dhaka South City Corporation removed around 1.5 tons of dead fish from Dhanmondi Lake on Friday.
Locals discovered thousands of dead fish floating in the lake earlier in the afternoon.
The authorities said no specific reasons for the fish's deaths could be identified immediately.
They fear that the number of dead fish may increase further on Saturday.
DSCC Public Relations Officer Md Abu Nasher said: "Samples of water and dead fish have been collected for analysis to determine the cause of the death."
He further said: "DSCC administrator also visited the site and instructed officials to remove the dead fish and conduct a thorough investigation into the matter.”
An official from DSCC said the fish might have been poisoned by someone with a vested interest, possibly a previous leaseholder of the lake who wanted to sabotage the current leaseholder's business.
"Another potential cause could be dumping illegal chemicals into the lake. Amid ongoing crackdowns in different areas of the country, those with such substances might have done it," the official added.
