Tests are being carried out on a water system at a Plymouth aquarium to see if it has been contaminated by diesel.
Several tonnes of red diesel flooded into the River Plym while a fishing vessel was refuelling at Victoria Wharf on Thursday morning.
Experts are going to be carrying out tests on a borehole which supplies river water to the National Marine Aquarium in Coxside.
Plymouth's marine laboratory is to work with the aquarium to analyse samples.
The initial results should be back by Friday evening, the laboratory said.
The leak was reported at about 0730 BST on Thursday.
It is not known exactly how much of the red diesel was released into the water, but the Environment Agency described it as a "large quantity and said it was investigating.
A clear-up has begun by the Cattewater Harbour Master.
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