Your fish most likely has Shimmy
Shimmy. Looks like the fish is swimming fast but staying in the same place. Shimmy is easy to spot, and if the fish is treated immediately, you can usually cure it and avoid more serious stress and disease.
It seems that livebearers such as mollys, platys and guppies get this disease more often than others.
Platys do not lay eggs they are livebearers.. which means the baby is live at birth. Also licebearers are better in groups of 3 females per 1 male.
Also corydoras are to be in groups of 6+ per species... I have mine in groups of three.
Treatment.
1. I would get Quick Cure to treat her. Treat once every day until the Signs of Stress and Disease are gone. On the back of the Quick Cure package it says, "Use one drop per gallon daily on all fish except those of the Tetra Group. For Tetra - Use 1 drop for every 2 gallons daily." The back of the package of Quick Cure says, "Remove charcoal or carbon from filtration system during treatment." But
www.aquariumfish.net tests show that leaving charcoal or carbon in the filtration system does not affect the treatment, so its recommended that you leave the charcoal or carbon in your filtration system, and leave the filter turned on and pumping during the treatment.
2. I would also add Aquarium salt. The dose recommended on the package label is one Tablespoon of Aquarium Salt per five gallons of aquarium water. So, for example, a 10-gallon aquarium gets 2 Tablespoons of Aquarium Salt. Do not repeat the salt and do not use iodized table salt.[/b]