THE cause of the 1500 dead fish at the Hoods Lagoon in Clermont earlier this week has been determined.

Isaac Regional Council Acting Chief Executive Officer Scott Riley said immediate investigations by the council this week have revealed the cause of dead fish at the western end of Hoods Lagoon, Clermont on Monday.

"The reason for this unfortunate incident has been the release of water into Hoods Lagoon from cleaning the Capricorn reservoir, which lowered oxygen levels in the Lagoon," Mr Riley said.

"Council apologises for the impact this water release has had on our environment and a natural community recreational area.

"The quick response by our Environmental Health Officers in aerating the water since midday on Monday restored oxygen levels, preventing any further risk to wildlife.

"We assure residents we're changing internal processes to prevent this from happening again.

"This includes carrying out comprehensive risk assessments and environmental management plans for future water releases.

"On first report of this incident on Monday, Council immediately contacted the Department of Environment and Heritage Protection (EHP).

"We are having active and open discussions with EHP, with our investigation to be submitted to the Department this week."