Understanding Erratic Swimming in Aquarium Fish

If you love aquarium fish, you know there are few things more enjoyably fascinating than watching them peacefully swim through their environment. The combination of their graceful movements and various speeds means that there is always something new to watch. But what if one of your fish seems to be swimming a bit differently than usual? Perhaps you’ve noticed that he starts to swim in one direction, only to jerk somewhat wildly away in another. Or maybe you’ve seen your fish brushing, banging or rubbing against the decorations or other objects in the aquarium.

While limited or isolated instances of these behaviors are probably harmless, frequent or repeated occurrences could be the sign of a problem—either with your fish or with the condition of your aquarium. Let’s take a look at some of the factors that can cause your fish to display these erratic behaviors:


The term flashing is the aquarium-speak definition of “itchy fish.” Essentially, when a fish is flashing, he is trying to scratch himself by rubbing or banging on things. There can be a number of reasons why a fish might flash, but parasite problems are a common cause and can be a good place to look first. Freshwater or saltwater Ich can cause flashing, so look for tiny white specks on your fish; this is the classic symptom of Ich and your fish may be flashing because of the irritation. Ich is a bit like fleas in mammals; the parasite has a somewhat complicated life cycle and recommended treatment entails a combination of natural remedies and quarantining.

Erratic swimming can also be caused by skin or gill flukes, which can be a little harder to detect. A fading in your fish’s color or change in the appearance of his gills—such as excessive visible mucus—can be indications of skin or gill flukes. If you suspect your fish is suffering from skin or gill flukes, it is best to take him to a veterinarian familiar with aquatic life for diagnosis and treatment. It can be treated with natural remedies if an outbreak occurs.

Water Quality

Erratic swimming can also be caused by poor water quality. Common culprits include incorrect pH levels (a pH of around 7 is usually correct although it can vary slightly from fish to fish), ammonia buildup, high amounts of nitrates or nitrites, or temperature changes. Regular water tests are essential, as water quality can drift in a matter of weeks—even sooner in a new aquarium that is being stocked with fish. An aquarium that is overstocked with too many fish can cause water quality problems that may be manifested by erratic swimming. Even a sudden change in water parameters can be problematic. Check your water frequently and anytime you add or remove fish from your aquarium.

You’ll also want to test your water whenever you medicate your fish, as some treatments can also have an effect on the aquarium’s bacteria colonies. If these are affected by the medication, the sudden loss of bacterial activity can trigger a change. Leftover amounts of chlorine or chloramines in your aquarium’s water can also cause erratic swimming, so it’s vital that you use the proper water conditioners whenever water is added to your aquarium. Routine water changes are important, as is keeping an eye on your filters and other aquarium components to make sure they stay in good working order to provide a happy, healthy environment for your fish family.