The group approached the reservoir with fanfare, carrying drums and pails of fish.
They were performing a tradition called fang sheng (releasing life), a gesture of compassion and repentance for sin during Vesak Day - but the practice is illegal.
Mr Ben Lee, 52, founder of outdoor exploration group Nature Trekker, recounts the incident two years ago at Lower Peirce Reservoir.
Mr Lee tells The New Paper: "I was alone and there were about 20 of them in the group. They wouldn't listen to my requests to stop.
"They had about 200 fish of different species with them and all the fish were released."
He was horrified as he knew the act could endanger the ecosystem.
Mr Lee and the Nature Trekker group have been patrolling parks and reservoirs for the last few years, especially around the Vesak Day period.
That is when instances of releasing animals tend to go up.
Though they have no enforcement powers, Mr Lee says they just want to do their part to educate people that the practice can do harm.
"We will come armed with cameras and tell them firmly that releasing these animals would upset the ecological balance of the habitat."
The group has been doing these patrols since 2007.
Mr Lee says the releases "are on a decline but still a concern."
The Government launched the annual Operation No Release in 2004 to spread awareness of the dangers of releasing animals into parks, nature reserves and reservoirs.
Over the last two weekends, the National Parks Board and the Public Utilities Board have been patrolling too.
According to a previous NParks advisory, about 80 to 90 per cent of released animals will die within a day.
A week before Vesak Day in 2004, rangers at the Central Catchment Nature Reserve found more than 100 dead quails. PHOTO: NParks
And if they survive, they could overwhelm the local animals.
National University of Singapore (NUS) ichthyologist Tan Heok Hui, who studies the ecology of non-native fish species in Singapore, says there are currently 92 non-native freshwater species recorded in reservoirs, canals and other water bodies.
Dr Tan has recorded 54 non-native fish species in Singapore's reservoirs. In the 1960s, there were only nine.
Out of the 54 species, 43 may have been introduced into the reservoirs via the release or discarding of unwanted aquarium fish.
The non-native species include the peacock bass and the giant snakehead. They are known to be invasive predators.
Dr Tan warned that these non-native fish could introduce diseases and exert a high degree of competition for resources.
"They could either wipe out the native species, or be eaten by native predators," he adds.
NUS researcher David Tan, 26, works at the Evolutionary Biology Laboratory and studies bird carcasses here.
He says released birds have a very poor chance of survival: "Most of these birds are captive creatures who have lost their instinct for survival.
"There are also a variety of predators such as eagles, owls, squirrels and even scavengers like crows, that will prey on them."
The scaly-breasted munia and Java sparrow are birds often released as part of fang sheng as they are common in the pet bird trade.
These birds are not native to Singapore, and their capture and trade fuels the decline of bird population in the wild, Mr Tan explains.
"It is actually not compassionate to release a captive bird because it is much more likely to have a shorter lifespan in the wild," Mr Tan says.