How big is the tank?
What is your ammonia level?
You ordered the guppies, so did they all come in the same bag? How long were they in the bag?
This is fin rot. Pretty common. Easy to treat but we need to determine the cause. Fin Rot is an indication of something else that is going on. My immediate suspicion is ammonia, i would rule that out first.
Do you have live plants and what other animals are in the tank?
I’m sorry i figured it out ammonia is fine have guppy grass in but I sell it so I make sure it is super clean and healthy. It is skin flukes.
I been treating them with general cure they are getting better
Ok my friend I seem to still need your help. I thought is was flukes so I did the full 5 day treatment of the general cure. Waited a day still her tail is disappearing and 2 have red gills. The red gills are throwing me off. Because I know 100% is not Ammonia. I did a 65% water change will gravel pump. And started pimafix treatment for dim rot day 3 of fin rot treatment and her tail is almost gone 2 have red fins still. Seems like if it is both fluke and rot the full flukes would have been cured with the general cure right? And them the pimafix would start on the rot. But no just worse. The babies are all fine I removed all the fish that wasn’t showing signs of un healthy. After the general cure treatment. They are fine with still no sickness. But my two adults I bought are very bad.
This is today
Fin rot can be bacterialorfungal. Pimafix treats just fungal. Melafix or API E.M. Erythromycin (powder packet) help bacterial. People have also said doing water changes and adding aquarium salt(one tablespoon per change) and stress coat for 10 days helps. And I've heard maracyn-two as well could work. Check some of the other fin rot posts on this forum for more information. I'm in no way an expert but this is what I've seen from other posts.
Guppies can handle a lot of salt and i wouldn't be afraid to use it in this case.
I'd do 1 tablespoon per 3 gallons.
They can handle it.
Dissolve it all first then add the entire salt solution a bit at a time over the course of about an hour or 2.