My koi keep flicking flashing?

Ooh, down a bit, down a bit, a little to the right, a little more, just a bit harder, yes that’s just the spot! Why is it that when we’ve got an ‘itch’ we just can’t reach it and no matter how well you direct someone else they have difficulty in finding it as well. Nevertheless, however small the itch or irritation is, it is so irresistible that we have just got to scratch it.

It’s often a mystery as to what makes us scratch, as there is rarely a visible cause. To others the sight of someone else scratching persistently can be both a source of irritation and a cause for concern for ‘the scratcher’ and depending on the cause, their own well being too.

Scratching is a very basic form of our body’s self-defence mechanism with our nervous system alerting us that there may be a ‘foreign body’ trying to enter our body through the skin.

We see the same response in cats, dogs, and other pets as well as fish. Flicking, scratching, scraping or flashing fish usually catch our attention through the corner or our eye as the reflective mirror-like flanks of the fish which are usually vertical, are thrown into the horizontal reflecting the daylight from above.

It is quite correct for us to be drawn to our koi or other pond fish showing these signs as this change in behaviour should ring some alarm bells, as we should always investigate further any changes from normal behaviour.

There are generally 3 possible causes for fish to flash, scratch or scrape and with a little further observation and your own detective work, you should be able to identify the cause and isolate the problem.


The most obvious cause for fish flicking is an irritation caused by external skin or body parasites. Just as in a cat or a dog (and sometimes even humans!) parasites are keen to enter or feed off skin causing an irritation and a scratching response.

The most reliable way of verifying whether the cause is parasitic is to take a skin scrape and view the sample of mucus under a microscope to view for any moving or obvious ‘creepy crawlies’.

How to take a mucus scrape.

Net the fish and bring the net to the pond edge

Gently raise the net out of the water until the top half of the fish’s body is exposed above the water’s surface.

In one gentle stroke, with a glass microscope slide between thumb and first finger, draw the edge of the slide along the exposed side of the fish in the direction from the head to the tail. Once you are satisfied that you have collected sufficient mucus, release the fish from the net.

Using a glass cover slip, spread the mucus into a central part of the slide ensuring it is wet. Squash down the mucus with the cover slip to form a thin film between the slip and the slide.

View the mucus under at least x100 magnification using a microscope and locate any moving ‘foreign bodies’ or stationary shapes that look ordered or structured among the random mass of mucus. This will confirm the presence of parasites. Using a good fish disease or koi handbook to confirm the exact type of parasite and its treatment.

After treating, take another sample from the same fish to determine whether the parasite population has been reduced.

NB. It is very rare to find a fish completely free of external parasites and any medication will not remove all offending parasites but reduce them to a level with which the fish is comfortable.

Parasites that may be seen include trichodina, chilodinella and the larger skin fluke. In fact the cause may even be larger parasites that are visible by the naked eye such as fish lice (Argulus) and anchor worm (Lernaea).

If any of these are evident upon closer inspection then it is more than likely that these are the cause of the fish’s irritation and must be eradicated using an appropriate medication.

Smaller microscopic parasites can be treated in situ in the pond using formalin and malachite treatments while nastier parasites such as the fish lice or anchor worm require nastier chemicals such as organophosphate preparations. However, these are quite controversial and carry health and safety risks to the user.

Short-term salt baths can be used at 4 oz/gallon however the fish must be netted an bowled and residual populations are likely to persist in the pond ready to re-infect the fish.

Poor Water Quality?

A second possible cause for irritation is poor water quality, particularly a build up nitrite which can be quite persistent. Nitrite toxicity can cause fish to flick and scratch through irritation, eyeing up a scratching post and flicking violently against it. Nitrite toxicity can also cause fish to jump clear of the water’s surface.

If you suspect poor water quality to be a source of irritation then simply test the water using a test kit. If the reading for nitrite is positive then there is a good chance of nitrite being the fish’s cause of irritation. The remedy is also straightforward. A 30% water change with no feeding until the nitrite is back down to zero and the flicking has stopped. It is prudent to try to locate the cause of the nitrite problem and it is usually a combination of too much food, too many fish and an immature biological filter. The build up of other pollutants such as pesticides or heavy metals can also cause a scratching response. It is wise if possible, to pass all incoming water through a purifier.

Just got to scratch!

The final cause for flicking is quite a natural one and that quite simply is that fish have an itch and need to scratch. One fish may have a loose scale or an unexplainable itch and flick and scratch to relieve the pain. Fish appear to be like humans in this respect and similarly as to when someone yawns, scratching too can be an infectious behaviour. In response to a single fish scratching, other fish too begin to scratch through an irresistible urge to follow suite. Have you tried this mind game while sitting on a train or waiting in a queue? Scratch you head or neck repeatedly and before long your actions will have caused others to uncontrollably follow! To satisfy yourself that the flicking fish are not irritated by parasites or polluted water just quietly watch them over a number of hours to see if one is a leader causing others to scratch and whether the scratching stops after a while.

As has been mentioned before, ‘fish watching’ is a key part of fish keeping, and is essential to be able to spot any changes in behaviour. If your fish do ever start flicking, with a little further detective work you should be able to determine whether it is a significant problem or not.

Kill blanketweed and string algae.