Flashing in Aquarium Fish

Bruising and scale loss secondary to fish flashing

Jessie Sanders

Flashing is a behavior fish utilize to itch their bodies, most commonly associated with parasitic infections. Since they do not have limbs or nails, they must use their environment to scratch themselves. You will often see your fish dart suddenly to the side or bottom of their tank and flatten out like a pancake, allowing their side to touch the substrate. Similar signs are fish who exhibit twitchy swimming or swim in sudden bursts. Some fish will jump out of the water and use the surface tension of the water surface to itch themselves. This is obviously very dangerous if they do not end up back in their tank.

If you do not see your fish actively flashing, you may see the secondary signs. Signs your fish are itchy include missing scales, bruising or trauma to the head and fins. If the trauma is severe enough, your fish may be susceptible to secondary infections and neurologic trauma. If left untreated, flashing fish are likely to get worse and can die from flashing too severely.

Causes of Flashing in Freshwater Fish

The most common cause of flashing is a parasitic infestation. The most common parasites in freshwater tanks that cause flashing are Trichodina, monogenean trematodes, also known as flukes, Costia or Icthyobodo, and Chilodonella. All of these are microscopic parasites and require a veterinarian to diagnose the correct parasite. The encysted stage of Ich is macroscopic but easily confused for fin ray fractures.

Primary parasite infections are common in fish tanks with recent additions of new fish or live plants without proper quarantine. However, outbreaks can occur in fish under chronic stress situations, such as poor water quality or overcrowding.

Fish commonly have a low level of parasites on them at all times, just given the nature of their aquatic habitat, but their immune system is able to keep the parasites in check. In a chronic stress situation, the immune system is weakened, allowing secondary invasions of parasites and bacteria.

Some fish are more sensitive to parasites than others. You may see one fish flash occasionally, but this usually does not indicate an outbreak. Multiple fish flashing or one fish flashing several times in a row are more concerning.

Temperature is critical to the progression of a parasite outbreak. The warmer the water, the faster the parasite life cycle and the faster your fish may become overwhelmed. If you suspect a parasite outbreak, do not adjust your temperature until you have consulted with your veterinarian.

Diagnostic Process

In order for your veterinarian to correctly diagnose the cause of your flashing freshwater fish, they will access your tank, perform water quality analysis and physical exams of your fish, likely under sedation. They may have recommendations to decrease the bioload or the total number of occupants of your tank, maintenance protocol recommendations, or tank improvements prior to laying hands on your fish.

During your fishes' physical exam, your veterinarian will take small skin mucus and gill samples. These samples will be evaluated under a microscope in order to determine if there is a parasite outbreak.


Treatment of flashing depends on the parasite causing the irritation. There are many treatment options to consider depending on the species of fish in your tank, additives already being added to your tank, the severity of the outbreak, and the status of the sick fish. There is no "one size fits all" treatment that will guarantee all your fish will magically be cured.

Some treatments may focus on correcting the primary stressor, such as poor water quality or overcrowding, rather than the parasite itself. In mild outbreaks, once the primary stressor is corrected, the fish will be able to take care of the infestation themselves.

How to Prevent Flashing in Freshwater Fish

The best method to prevent a parasite outbreak in your freshwater tank is to quarantine all new fish and live plant additions for 4-6 weeks prior to adding them to your tank. This will isolate any potential sick fish and prevent many diseases, including parasites, bacteria and viruses, from entering your healthy system. If your new fish gets sick, they can be quickly and effectively treated. Quarantine plants without live fish to break the parasite life cycle.

If you see your fish flash only occasionally, you have not added any new fish or live plants in the last 3 months, your water quality is within range, and you are feeding a good diet, do not be too concerned. Sometimes, a fish may just flash for something to do, or has a very low level of parasites they are able to clear on their own. Make a note of which fish you saw flash and when and monitor for an increase in severity and any injuries.