From The Aquarium Wiki
Shimmies or 'Shimmers', also known as "Molly Disease" or "Livebearer Disease", is most commonly seen in Mollies or Guppies. It gets it's name from the action the fish does, which is to stay relatively still and wag, or "shimmy" its body from side to side, and often includes the fish scratching and itching on objects in its environment.
Causes of Shimmies
Despite being labelled a disease, it is not a disease at all but the lack of electrolytes in the water that prevents the fish's metabolism from working correctly. If left untreated this can be fatal.
Curing the Shimmies
Electrolytes can be added to the water via Salt (add 1% solution), and it's the lack of this in the water that most often causes the Shimmies in Mollies, some species of which are essentially brackish fish and not freshwater. All pet shop Mollies tend to be hybrids, so some will be fine in freshwater, some need a more salty environment.
Check first that there are no other species of animal that may be harmed by adding salt.
Alternatively if your Mollies are captive bred then it is more likely the water isn't hard enough. Ensure the GH, KH levels are at least 8 degrees and pH is within the range of the species affected. Add some sodium bicarbonate or powdered coral sand to increase these levels.
There are several products on the market that are designed to help cure the Shimmies, even though the addition of the correct type of Salt should suffice. These include the following:
- Molly Bright - Liquid Treatment by Jungle Laboratories.
- Shimmy Blocks
- Interpet - No. 5 General Tonic