The Shimmies in Mollies, Platies, and Guppies: Causes and Fixes

When a livebearer rocks or wobbles in place without swimming forward, it's usually telling you its water chemistry doesn't match its needs. Here's what causes 'the shimmies' and how to correct it.

"The shimmies" is the common name for a livebearer — most often a molly — rocking or wobbling from side to side while making little or no forward progress. It is not a single named disease but a symptom: a sign the fish has lost some normal control of swimming, most commonly traced back to water chemistry that doesn't match what the species needs.

Water Hardness and pH: The Most Common Trigger

Species profiles for two commonly kept mollies make the same point. For Poecilia sphenops (short-finned molly), Seriously Fish states the species 'must be maintained in moderately hard or harder water, with a basic pH,' explaining that it is the 'hard' minerals — calcium and magnesium — that are 'crucial to the long-term health of this species,' and that 'when kept in soft or acidic water, the fish weakens fairly rapidly, frequently indicated by shimmying, fungus, and/or clamped fins.' The profile for Poecilia latipinna (sailfin molly) describes the identical pattern in nearly identical wording. This lines up with the more general principle from the Merck Veterinary Manual that 'fish absorb minerals from the water column; therefore, use of water with very low [total hardness] can result in poor growth and mortality.'

Target Ranges for Common Mollies

FishBase lists an overlapping range for Poecilia sphenops (pH 7.5–8.2, hardness 11–30 dH), consistent with the 'moderately hard, basic-pH' requirement described above.

Other Contributing Causes

• Cold water or a sudden temperature drop — fish are cold-blooded, and all physiological processes, including nerve and muscle function, are governed by water temperature
• Generally poor or unstable water quality, including ammonia buildup
• Protozoan skin/gill parasites such as Ichthyobodo, which attack skin and gills and can weaken swimming
• Columnaris (Flavobacterium columnare), a bacterial disease common in warmwater fish that produces skin and gill lesions
• Cumulative stress from any of the above, which the Merck Veterinary Manual notes can compromise immunity even from temperature shifts of just a few degrees

Correcting the Shimmies

1. Test general hardness and, if low, raise it toward the moderately-hard-to-hard range these species need; confirm pH sits on the basic side (around 7.0–8.5) rather than acidic
2. Confirm temperature is within the accepted range for the species and avoid rapid swings; general guidance for changing fish water temperature is no faster than about 1°F (roughly 1°C) per hour
3. Perform a water change and confirm ammonia and nitrite are at zero
4. Watch for accompanying fungus, clamped fins, or skin/gill lesions, which point toward an infectious cause such as Columnaris rather than water chemistry alone, and warrant closer inspection

It's also worth knowing that Seriously Fish notes 'years of inbreeding and over-production have resulted in many of the mollies available in the hobby today being very weak genetically and prone to disease' — meaning some individual fish may shimmy or struggle even in well-maintained water, simply because they started out fragile.

Sources: Seriously Fish, Poecilia sphenops (www.seriouslyfish.com); Seriously Fish, Poecilia latipinna (www.seriouslyfish.com); FishBase, Poecilia sphenops (www.fishbase.se); Merck Veterinary Manual, Environmental Diseases of Aquatic Animals in Aquatic Systems (www.merckvetmanual.com); Merck Veterinary Manual, Bacterial Diseases of Fish (www.merckvetmanual.com); Merck Veterinary Manual, Parasitic Diseases of Fish (www.merckvetmanual.com).