Auckland Council is pleading locals to stop releasing goldfish into local lakes and streams as the fish and plants play havoc with the water quality.
Goldfish have turned up in ponds and streams around the city, and they're not swimming there by themselves.
Peter Wilcox has been helping Auckland Council pull invasive koi carp out of local streams and ponds, and he has been catching another type of fish along the way - goldfish.
But the fish he was catching were not like the ones in pet stores.
"Western Springs, the last catch the smallest was 30 centimetres, the top was 40 centimetres, and they ranged in weight from just under a kilo to 1.8kg."
"A goldfish that might be worth $10 in a shop becomes thousands of dollars expense for the councils trying to manage it" - Peter Wilcox
Wilcox said the fish aren't swimming there by themselves, it's more likely they've come from aquariums tipped into local waterways.
"A goldfish that might be worth $10 in a shop becomes a thousands and thousands of dollars expense for the councils trying to manage it so they can manage the amenity for the good of the rest of the population. It's irresponsible, it's effectively environmental vandalism."
Goldfish can be bought for as low as $4.50 in pet stores, but their tank requirements and maintenance can surprise owners.
Hollywood Fish Farm Mt Roskill store owner Vicky Ferguson has seen it before.
"Because they're purchased at quite a small size people think they can put a lot of them into a tank, and then they start to grow a lot bigger and outgrow the tank and cause a lot of mess. That's when you get into issues with high waste levels in the water which can cause disease."
Goldfish at Hollywood Fish Farm. Photo: RNZ/Katie Fitzgerald
Councils remove goldfish because when they get into lakes and streams they rip up plants, outcompete native fish for food and stir up sediment to make the water cloudy.
Auckland Council freshwater ecologist Belinda Studholme said it was not the fish that was a problem, the plants that came in tanks were an issue too.
At the Paremata Ponds in Henderson Cabomba also known as fanwort, and parrots feather have both taken off inside the stormwater system.
"They totally destroy water quality, they change the dissolved oxygen and pH and the ponds. They exclude all other native vegetation and make the pond unsuitable for native fish."
Studholme said outside of stormwater ponds weeds like cabomba and parrots feather made for pretty gross swimming, and caused trouble for native fish.
"We do have a lot of native species like charophytes and native macrophytes that are quite sort of delicate, and provide the sort of habitat that that our native species need, kōura, freshwater crayfish and freshwater mussels. They just wouldn't be able to survive when these sorts of weeds take over."
Removing the weeds is proving tricky, but over in Western Springs, Wilcox is having success with removing the goldfish.
He said there was only one option once they were out of the water though.
"To relocate them is a major issue from biosecurity because we don't know what diseases this population have, what else will travel with them. So basically, the only thing we can do is to euthanise them."
Ferguson said owners out of their depth could surrender them, or find a new home for them with somebody with a larger tank or pond in their home.