Could You Please Explain What Flashing Is?

Could You Please Explain What Flashing Is?

My Koi fish will occasionally jump or do little twists in the water….I always thought they were just playing….is this the sign of a disease though? I have an 8000 gallon pond with superb water quality…..thanks Paul C.

Flashing is a condition in which fish quickly lay over, then dash their sides against something in the pond. Often, they are trying to dislodge a parasite from their surfaces. In many other instances, they have been put into new water, water with a new pH, water with some Chlorine, or water with some other chemical irritant (High Ammonia levels for example.)
Flashing is normal in fish, when you see it once or twice per day per fish that would be ‘okay’ but if you see fish flashing two at a time in a pond it’s more than coincidence and water quality, and CHANGES in water quality should be evaluated. For some people, it’s simply the daily fluctuations in pH that cause the problem.  In other cases it’s a parasitic issue. Several parasites cause profound flashing including but not limited to Costia, Flukes and Trichodina.

Could You Please Explain What Flashing Is
Could You Please Explain What Flashing Is?

Could You Please Explain What Flashing Is?

Fish’ll ‘flash’ and scratch when they have Flukes, Trichodina, or an early Anchor worm infestation. Almost any parasitism can cause fish to flash or scratch on things. One problem is that the parasites can damage the skin, opening it to infection. The other problem is that the fish can damage their own skin when they flash, so you need to get them to stop.

A great shotgun for flashing Koi would be a moderate salt level and Praziquantel. Ignore me, if you have live plants. Salt kills live plants.

Another important realization is that rapid changes in water quality can cause flashing. A large freshwater change can cause flashing as the fishes’ skin adapts to a slightly higher or lower pH. At the end of the day my pond has a falling pH as the algae bring the pH down a bit. I’ll see some flashing then. Residual chlorine or other metals in new water can cause flashing. Using a dechlorinator binds these metals and alleviates a lot of the scratching. Koi, goldfish and pond fish brought to shows can flash like crazy. The new water in a show tank can really irritate the fish, causing flashing.  Still, you have to keep in mind that a parasite could potentially be causing the flashing, so caution and quarantine would be a great idea.

Finally, black plastic water lines that contain large amounts of UV stabilizers and elasticizers can set up an irritation sufficient to cause excessive flashing. Never use a novel plastic with your fish. Always use things which you know won’t hurt fish.

Check your water and consider a biopsy for parasites – or an empirical treatment.


Dr Erik Johnson is a Marietta, Georgia Veterinarian with a practice in small animal medicine. He graduated from University of Georgia with his Doctorate in 1991. Dr Johnson is the author of several texts on Koi and Pond Fish Health and Disease as well as numerous articles on dog and cat health topics.