[header] How to treat fin rot [title] Take the affected fish out of the tank. [step] Start by removing the affected fish from the tank and placing him in a separate tank with clean, fresh unchlorinated water. [substeps] You will also need to remove the other fish in the tank to a separate container with clean, fresh unchlorinated water. 
Use a different net to remove the other fish as fish rot can be spread through shared nets. Do not place the affected fish in the same container as the other fish as this can cause the fin rot to spread.