Almost 40 fish have become the suspected casualties of an effort by Hastings District Council to prevent an outbreak of E. coli in Havelock North.
The council added low levels of chlorine to the town's water last week, when two back-to-back tests confirmed traces of the bacteria.
However, this precautionary measure has meant resident Gary Fargher has buried about half of his $800 school of fish.
"It is a damn shame - I am losing them left, right and centre at the moment," he said.
"I have buried 18 so far and the others don't look too good.
"I saw yesterday that they were looking a bit sick and weren't feeding, and when I got up this morning to go and check them half of them were on the bottom of the pond dead. It wasn't very pleasant, some of those bigger ones are probably 18 years old at least."
Mr Fargher said he believes the damage was done when he decided to give his pond a bit of clean out on Saturday before he went to the races.
He said he did not know about the chlorine in the water until yesterday morning.
"I put the hose in the pond on about three-quarter strength and left it running until I got home," he said.
"So it had a good dose of chlorine in it, and by then it was just too late to do anything about it."
Mr Fargher said the only thing that changed in his pond's water was the chlorine added to the water.
"It has to be the chlorine, I can't put it down to anything else," he said.
HDC water services manager Brett Chapman said council's main focus had to be on public health and safety.
"That means that in the situation we had late last week chlorine must be introduced," he said.
"Having said that, council acknowledges that fish have a particular sensitivity to chlorine and in the case of any incident discusses the outcome with an affected resident on a case-by-case basis. Other animals [and] pets will be unaffected."
Mr Fargher said while he understands council cannot do anything - he said perhaps staff could work with fish owners to develop a register to keep them informed of any water changes in the future.
The council said there had been no reports of any E. coli type illnesses.
Mr Chapman added while the source of the contamination had not been detected, the No3 bore in Brookfields Rd was under suspicion and had been removed from the network.
"The matter is still being investigated [and the bore] will not be reintegrated into the system until the council and the Hawke's Bay District Health Board are completely assured that it is secure, [and] safe from surface contamination," he said.
He said another possible source of the bacteria is the heavy rain the district recently experienced and as such the storm water is also a part of its investigation.
"The latest tests are completely clear of E. coli - the third day of clear tests in a row that is since Friday morning," he said.
As tests have come back clear three days in a row, Mr Chapman said the treatment of water can now cease.
"Council has stopped using chlorine in the supply, however it may take a couple of days before the taste completely disappears."