According to the South East Coast Ambulance Service, they were called at around 8.30am this morning (June 10) to Alderwood Ponds in Horsham Road, Steyning.
A man had reportedly collapsed and fallen into the water.
Ambulance crews were sent to the scene, along with the air ambulance.
Paramedics had to give the man emergency treatment at the scene, and he was then taken to Worthing Hospital by road.
The ambulance service spokesman said the man was in a serious condition when they left the scene just before 9.30am.
Alderwood Ponds are popular for coarse fishing, which means anglers will catch the fish in fresh water and then release it back into its habitat.
Angling was allowed to resume on May 13 as lockdown measures were eased.