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Overcoming The Shimmies In Tropical Fish

While most associated with mollies and guppies, the 'shimmies' can affect many other types of fish as well.

Shimmying is a symptom rather than a single disease, and an indication that a fish no longer has proper control of its nerves and muscles. Mollies and guppies are known to display the shimmies. Via Marrabbio2/Wikipedia

Fish suffering from the shimmies rock from side to side, an action known as shimmying. There are often other odd postural or behavioral traits associated with the shimmies, including clamped fins, head shaking, yawning, and labored or heavy breathing.


Because shimmying is a symptom rather than a specific disease, it can occur alongside other symptoms as well, particularly ones associated with stress, poisoning and water chemistry problems, such as excessive slime production, cloudy eyes, bloating and lethargy.

Although some fish may twitch, vibrate, or shake their bodies for short periods, particularly when exhibiting aggressive or reproductive behaviors, no fish will continually do so. This makes positive identification of shimmying relatively easy.


Shimmying is a symptom rather than a single disease, and an indication that a fish no longer has proper control of its nerves and muscles. It occurs when fish are under severe stress, most often because of environmental problems.

The classic scenario is when mollies are kept in soft or acidic water conditions. Though tolerant fish in many ways, they do not do well in soft or acidic water, and it is very common to see mollies kept that way start to shimmy. To varying degrees, almost all the other livebearers sold to hobbyists are sensitive to soft or acidic water conditions, and consequently the shimmies may be seen among any of them kept in the wrong water conditions.

Cold water conditions can have the same effect on tropical fish as well, with cichlids in particular being extremely sensitive to even short-term exposure to cold water. Poor water quality can also trigger the shimmies, as can other sorts of poisons such as copper. Certain bacterial and protozoan diseases, such as finrot, mouth fungus,  and slime disease, seem to be associated with the shimmies as well.


There is no treatment for the shimmies as such, but once environmental conditions improve, affected fish usually get better without problems. It is important not to try and change things too quickly though, as sudden changes, even to the better, can be so stressful that they end up making things worse!

So, in the case of an aquarium where the water chemistry was wrong, a series of small water changes would need to be performed to gradually change the ambient water chemistry conditions across several days or a week. The bigger the intended change, the more gradual the changes will need to be. A good approach is to change no more than 10% of the aquarium per day, and for a few hours afterwards, keep a close eye on the aquarium to make sure all the fishes inside are behaving normally and showing their usual level of interest in food and one another.

Similarly, if fish have been chilled, perhaps by transportation for many hours on one of the coldest days of the year, then their re-acclimation to warmer water conditions should be done very carefully. Don’t dump the fish straight into the aquarium, even though you know water quality is going to be much better in the aquarium than in the bag. Instead, empty the bags into a bucket, and then use some variation of the drip method to add small portions of aquarium water into the bucket across 30-60 minutes. Only then is it safe to net the fish out and release it into the aquarium.

Prevention: Community Fish In General

Given that the shimmies is a reaction to environmental stress, the best way to prevent this problem is simply to ensure all fish are kept in appropriate environmental conditions. In particular, pay close attention to water chemistry and temperature demands. This information can be obtained from any good aquarium fish encyclopedia. In addition, always ensure that ammonia and nitrite levels are kept at zero, and that nitrate levels are as low as possible, ideally 20 mg/l or less. Good filtration and regular water changes should take care of this.

Prevention: Mollies And Guppies

Mollies  and to some degree guppies present problems for casual aquarists because their demands are not entirely recognized by the hobby. The fancy guppies sold in most pet stores are demonstrably more delicate than wild guppies, non-pedigree (‘feeder’) guppies, or even the fancy guppies kept by hobbyists 30 or 40 years ago. Modern fancy guppies must be kept in very well-maintained, spotlessly clean aquaria with warm water and moderately hard to hard, basic water conditions, i.e., 75-82°F, 10-25°dH, pH 7.5-8.0.

Aquarium strains of mollies are even more demanding. To begin with, they must have hard, alkaline water conditions; 15-30°dH, pH 7.5-8.5. A fair degree of warmth is also important, i.e., 75-82°F. But they must also have very low nitrate levels too, less than 20 mg/l, and absolutely no ammonia or nitrite. This makes them completely unsuitable for small, overcrowded tanks or really any sort of aquarium that doesn’t have a well-maintained and mature biological filter. Hobbyists argue whether the addition of 1-3 teaspoons of aquarium salt per gallon is absolutely essential to keeping mollies, but it certainly makes them easier to keep. They do even better kept in a proper brackish water aquarium.

Want to learn more about tropical fish diseases? Click here for the most common diseases that affect fish.

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Fish · Health and Care