Why is My Fish Swimming Fast Back and Forth?

While it is common for your fish to sometimes swim back and forth, it can become a problem if this happens a lot. This weird swimming behavior can be accompanied by your fish swimming up and down, rubbing themselves against gravel or plant decorations. Some fish can crash against the aquarium walls. All these are different types of behavior that are generally caused by stress. 

Poor water conditions or improper feeding habits can cause your fish stress. To deal with the behavior it’s important to identify the reason that is causing your fish to swim this way.

Poor Water Conditions

If the water conditions inside your tank are poor, your fish can become stressed. High levels of ammonia and nitrites, low oxygen levels, extreme pH levels in the water can cause the behavior. The water temperature not being within the correct range can also be causing stress. For saltwater tanks, extremely low or high salinity levels could be the cause.

To find out if this is the cause, test the water with an aquarium testing kit. Use the testing equipment to measure the level of toxic substances like ammonia, nitrates and nitrites. If they are high, perform a 20 percent water change to stabilize the water. If the levels are still high, wait 2 days and then do another 20 percent water change to help balance everything.

The water temperature should also be checked. Your fish can become stressed if the temperature is not in the correct range. If the water temperature is not correct, check to make sure that the aquarium heater is functioning properly. If the heater has malfunctioned it should be replaced so that a consistent temperature can be maintained. 

Improving the water conditions should prevent your fish from swimming fast back and forth.

Compatibility with Other Tank Mates

Not all types of fish can get along with each other. If you are a new aquarium owner, you may have accidentally added incompatible fish in the same tank. Their incompatibility can cause aggressive fish species to nip the fins of peaceful fish species.

This behavior will stress the peaceful fish and make them swim in a strange pattern to protect themselves. You can place the fish that is showing weird swimming behavior in another tank. If moving them to another tank stops the behavior, it means they had been getting stressed from other fish. If this is the case we recommend keeping them in a different tank.


Another similar reason that can cause your fish to become stressed is if your aquarium becomes overcrowded.  Having too many fish in a small tank can lead to a higher bioload. High bioloads can cause ammonia spikes that can stress your fish. 

If you have another aquarium you can try moving some fish to another tank to minimize the bioload. Performing a 25 percent water change should also reduce the level of toxic substances that could be causing your fish to stress.

Having a higher bioload is not the only reason why overcrowding is a bad thing. Just like with people, fish like to have some personal space. Having too many fish in a small area forces your fish closer together causing a lot of unwanted stress.

Improper Feeding

Feeding your fish less food could also be one of the reasons for the fish to become stressed. Less food could cause your fish to not get enough to eat. At the same time they should not be fed a lot more than what they can eat. Uneaten food can cause high levels of toxic chemicals like ammonia inside your aquarium. Feed them only what they can eat within a few minutes of being fed.