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11.7. Shimmying

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Author : David Bogert

Published :

Time To Read :
3 minutes
11.7 Shimmying in Tropical Fish
11.7 Shimmying in Tropical Fish

Fish suffering from the shimmies rapidly quiver as they swim, rocking from side to side. 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.

All of the following can cause shimmies:

  • Genetics (especially the molly)
  • An internal bacterial infection of the brain
  • Chlorine in the change water
  • Very poor water quality
  • Aerosols such as disinfectants or floor cleaners close to the fish tank
  • High bacteria count in the water
  • Bacterial toxins from a decaying fish or fish food

A common cause of shimmies is chlorine in the water from a super chlorination event. Since this chlorine caused shimmying will occur concurrent with a large water change many hobbyists come to the conclusion that a large water change will cause shimmies. The old saying is “correlation is not causation”.  A large water change is NOT a cause for shimmying.  One can do a 100% water change with impunity as long as all the chlorine has been neutralized (with the notable exception of well water, which often has no oxygen in it and can suffocate fish).

Mollies are especially prone to the shimmies because of genetics. Mollies have been bred from many distinct species. Then they have been repeatedly line bred. As a result Mollies tend to “break down” in their genetics. So mollies are very genetically prone to the “shimmies”. This genetic “break down” also occurs in guppies, a species which has a broad distribution in nature. The other livebearers, platies and swordtails, are all descended from a single species which a relatively small distribution. So they don’t “break down” when line bred.

The male molly shimmies in the courtship display. In mollies which have been line bred too much this seems to be amplified to the point where the fish can no longer control themselves. Both male and female mollies start shimmying in place in the aquarium. There is little one can do for such genetically flawed fish.

Poecilia sphenops, Dalmation Molly
Poecilia sphenops, Dalmation Molly

Many claim that soft, acid water causes shimmies in mollies. There is no science that backs this claim. But it won’t hurt to add one teaspoon baking soda and one tablespoon salt per ten gallons to the aquarium where the platies or mollies has the shimmies. Some crushed coral in the filter will harden the water. Call it a placebo for the fish hobbyist.


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