Normal aquarium lights don't have Ultra Violet (U/V) light in them so you don't need to turn your lights off when treating.
Do a big water change and gravel clean the substrate 24-48 hours after treating them.
If you removed the carbon cartridge from the filter and it has dried out, throw it away and don't bother re-using it. Once carbon has been wet and allowed to dry, it is no longer any good.
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Normally fish that are infected with a few intestinal worms will lose weight. But if the fish has lots of worms in its body, the fish can swell up and look like a pregnant guppy (or like your fish). This is due to the huge number of worms in the body.
Once the worms have been killed (this normally takes a couple of hours), the fish poop out the worms and it will look skinny after the worms have been expelled. But you leave the medication in the tank for at least 24 hours.