Formalin is a popular parasitic tretament for fish, often used alongside malachite.
Check your water quality before adding any treatment to the water.
What Will Cloverleaf Formalin Answer Treat
Dosage rate of Cloverleaf Formalin Answer
Do not add any other medication to the pond for at least 10 days after the last treatment
Do not use if water temperature is below 10 degrees