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I have a 8 month old pond, app. 2000 gallons, with a plastic liner. One of my Koi has developed a white fungus or mold on its mouth. It looks like a white mustache. The other 5 fish, all about 5" long, appear to be normal. Also a stringy hair like algi is growing on the sides of the pond and flowing from the water fall. The water is very clear. I am using a biological filter.

Will a rock salt treatment help and, if so, at what rate? Thanks for your help. BM


Your best first step is to test your water from top to bottom, it's easy and it's not expensive. Ammonia, Nitrite, Nitrate, pH are good starting points.

Second - The green algae you see is string algae, I have it i my pond, it's a "good thing" and suggests that your pond is at a sort of equillibrium.

Third: The white mustache could be mouth rot, but it could also be little more than a pox virus, just waiting for Spring to warm up the pond, so the virus can go away. This is especially suspected if the little affected fish is eating and otherwise happy.

If the affected fish is valued, consider setting up an indoor holding system. Search info: BIOSEEDING to establish a quick equillibrium in the tank. Warm the fish slowly to room temperature and then slowly up to 72-74 DF - then see if any treatment at all is indicated.

If it's mouth rot, injection of antibioitics along with a couple of swabs with hydrogen peroxide would be commendable in a warmed environment.

Mouth rot is usually noted as a depressed fish with red or bloody lesions adjacent to the affected lip or jaw.

Don't go crazy with medicines in the main pond.


Fish disease problems? Need some help? The help pages take you through the basics of the 'fish health work-up' diagnosis method and basic fish disease treatment methods.

90% of all health problems and fish deaths are caused by poor fish keeping! The key pages are essential reading for all beginners and intermediates who are serious about disease prevention.

Expanded Content by Dr. Erik Johnson, and Used with Permission; Frank Prince-Iles ©2009 All Rights Reserved