Around 1,200 fish have died in a lake at a popular tourist spot.
The Environment Agency is testing some of the dead fish from Aberdare Park lake in the Cynon Valley, and results are due shortly.
The park is still open to the public but visitors are being asked not to touch the dead fish.
A spokesperson for Rhondda Cynon Taf council, which runs the park, said: "We've not had any previous problems of this kind at the lake."
Environment Agency Wales has taken both live and dead fish away for testing.
"Whatever we find out, we will be giving the council some advice about what they can do," said an Environment Agency Wales spokesperson.
Different species of fish in the lake have been affected including carp, goldfish, rudd and roach, but some fish have survived.
A spokesperson for Rhondda Cynon Taf council said: "There are signs on the site advising people of the diseased fish in the lake and advising people not to touch the dead fish.
"The Environment Agency is carrying out the tests and we're speaking to them again today.
"They've taken away 20 fish for testing, both dead and alive."
Some residents who live near the park said action was needed as quickly as possible.
Margaret Pype said: "It's a lovely park and it's kept in good condition.
"I go up the park every day with my dog and there's a notice up saying the fish are infected. Why don't the council do something about it?"
Caroline Lawrence, who lives by the park, said: "I think it's dreadful because it's such a lovely pond.
"But fish can't live in filthy water and it is filthy."
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