Melafix is not gonna stop any serious bacterial infection nor camallanus worms.
You need to just
A. Euthanize the poor fish and start over, bleach the tank
B. If its only worms, order some powder levimasole and use epsom salt at high enough concentration to get them to pass the worms, but be careful because sodium magnesium is only accepted by the fish for like a few days at a time, so maybe extended baths in it.
Dont feed anything besides the ocassional peas or algae ruffage, and MAYBE they have a chance.
I know there has been alot of internal type stuff going around lately, its a big reason i dont have guppy fish right now.
They are continuing to use de-wormers pretty frequently and when you get ffish if there is even a hint of a nematode in the substrate or whereever, they will manage to get into the fish. The worms and other stuff becomes immune to the medicine while the fish become dependant on it to survive because there is no survival of the fittest on the scale there would be in nature....
I reccomend starting over, bleaching everything and scrubbing thuroghly, including/especially the siphon tube and buckets and filters
If you just deworm the fish thats great but the worms will probably keep coming back until its been nuked with bleach.
I know it bites but not much we can do.
In the future, get guppies from a reputable breeder, ya its fun (and cheap)
to mess around with petsore fish and thats fine, i did it for years, but they are just becoming too sickly nowadays alot of petstore tanks are swarming with disease and parasites because they use gravel and rarely clean the tanks completely.
Buy from a reputable guppy breeder and save those fish keep them seperate M/F and dont breed brother to sister only Daughter or son to parents once or twice then get fresh blood of the same color or keep two seperate lines.
You may not be that seroius about guppies, but i figure its about the only way to get healthy fish nowadays.
Walmart, petco, and petsmart, heck most mom and pa stores, have parasites in their systems that will wipe out your tanks.