The fish is probably just fat. If it's still eating well and doing normal poop, then do not treat it with anything.
Never use medications (especially anti-biotics) unless you know what the fish has.
Chemicals harm the fish as well as kill disease organisms in the water. If you add chemicals and there is nothing wrong with the fish, you are harming it for no reason.
If the fish is eating well and doing normal poop, and you haven't opened the medication, then return the medication and get a refund.
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Livebearers like guppies, mollies, swordtails & platies are regularly infected with gill flukes and intestinal worms.
You can use Praziquantel to treat tapeworm and gill flukes. And Levamisole to treat thread/ round worms.
Remove carbon from filters before treatment and increase aeration/ surface turbulence to maximise oxygen levels in the water.
You treat the fish once a week for 3-4 weeks. The first treatment will kill any worms in the fish. The second and third treatments kill any baby worms that hatch from eggs inside the fish's digestive tract.
You do a 75% water change and complete gravel clean 24-48 hours after treatment.
Treat every fish tank in the house at the same time.
Do not use the 2 medications together. If you want to treat both medications in a short space of time, use Praziquantel on day one. Do a 75% water change and gravel clean the substrate on day 2 & 3. Treat the tank with Levamisole on day 4 and do a 75% water change and gravel clean on day 5, 6 & 7 and then start with Praziquantel again on day 8.
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If you want to treat the fish for anything, raise the temperature to 30C (86F) and keep it there for 2 weeks. The high temperature will kill any whitespot parasite that might be causing the fish to rub on objects. You can add some salt too. Salt will kill other types of protozoans that don't die from heat.
After the salt and heat treatment, I would deworm all your fish at the same time. If you have other tanks and share nets, etc, you should probably raise the temperature 30C (86F) on all the tanks for 2 weeks.
You can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres of water. If there is no improvement after 48 hours you can double that dose rate so there is 2 heaped tablespoons of salt per 20 litres.
If you only have livebearers (guppies, platies, swordtails, mollies), goldfish or rainbowfish in the tank you can double that dose rate, so you would add 2 heaped tablespoons per 20 litres and if there is no improvement after 48 hours, then increase it so there is a total of 4 heaped tablespoons of salt per 20 litres.
Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.
The salt will not affect the beneficial filter bacteria but the higher dose rate will affect some plants. The lower dose rate will not affect plants.
After you use salt and the fish have recovered, you do a 10% water change each day for a week. Then do a 20% water change each day for a week. Then you can do bigger water changes after that.
