Oxygen levels in Llyn Padarn at Llanberis may become critically low endangering survival of the rare arctic char.
Environment Agency Wales (EAW) officials have drawn up plans to remove the fish and re-stock the lake if that is necessary.
A spokesman said: "We are continuing to take oxygen, temperature, pH and other chemical measurements on a monthly basis to build a full picture of what is happening in the lake.
“Char were previously translocated from Llyn Peris during the construction of the hydro electric scheme.
“We are in discussion with CCW over the feasibility of rearing and translocating char from Padarn if this becomes necessary. We will need to agree a location and determine that the receiving lake is suitable and that we don’t cause issues for existing species."
The arctic char, or torgoch, is found only in Llyn Padarn in Wales although it is more common in Scotland and Scandinavian countries.
Its existence has caused the lake and its surroundings to be denoted as a site of special scientific interest (SSSI).
But the popular tourist lake has been blighted by the potentially toxic blue-green algae throughout the summer with people and pets banned from swimming in the water for their own safety.
Anglers believe discharges from a sewage treatment works into the lake are to blame. They claim the numbers of fish caught in the lake and nearby rivers have plummeted.
Countryside Council of Wales’ experts are equally concerned and have called for urgent action.
But a new forum, the Llyn Padarn Stakeholder Group formed to bring all interested parties together to discuss the algae issue, have been told tests on samples taken from the village’s sewage works showed it was operating within its consent limits.
But an Environment Agency Wales spokesman admitted the maximum allowable phosphate for this year had been reached and had to be reduced further.
To prevent the problem flaring up again Environment Agency Wales wants to drive down the amount of nutrients entering the lake from locals’ washing machines and dishwashers.
Algae feed on nutrients in the water which reduces the amount of food available can help to combat the growth of algal blooms in the lake.
A campaign drawn up by EAW officials aims to persuade locals to only use washing machines and dishwashers when full or use a detergent with reduced or no phosphate.
“We’re asking the people of Llanberis to help because this is very much about improving the quality of their environment and helping the local economy,” the EAW spokesman said.
Businesses want more urgent action to resolve the problem before it affects the tourism industry.
Algae Threat to Rare Fish in Llyn Padarn
UK - A rare fish may have to be moved from a North Wales lake unless a problem with algae is resolved.