Sick Guppy

[golden_wire_fox]

Hi, I'm new here, but I thought I should make an acount here sence I am starting to get some tropical fish.
I've always had betas and goldfish, I was very good with them, they were my little babies, but along the way I got a high fin tetra. Then before I knew it I got 8 fish that is not a beta or a goldfish.

So the day before yesterday I did a half tank water change on my 20 gallon and I added Quick Start and Slime Coat to the water. (I add AquaSafe to my water every month as well, but I did not add it during the water change because I just added that 2 weeks ago.) Well I thought it would be exeptable to add a cute guppy to my tank sence I have it all cleaned up. So I went and I bought one male fancy guppy.
When I put him in the tank all my fish was totally chill with him, they let him eat with them and he played with my molly fish so I thought everything was fine. Then I woke up this morning and his tail looks like he has a bad case of fin rot, he is only using one of his side fins to swim, and he stays at the top of the tank out in the open. He doesn't look like he is gasping for air, he is just setting there and all the other fish looks puzzled as to what is his problem. They are not trying to hurt him so I don't think they ripped his tail.
Well sence I thought he has fin rot I put some All In One LifeGuard in the tank.
I feel like I'm probably being a failure of a fish mom right now so can anyone help me?

 

[golden_wire_fox]

Here is a picture of him, before and after.

 

[FroFro]

(I add AquaSafe to my water every month as well, but I did not add it during the water change because I just added that 2 weeks ago.)

Are you saying you only put water conditioner in the tank ONCE a month? You DO use it when you replace the water that you removed during cleaning right? You replace the removed tank water with conditioned water right? As for his tail I've not heard of fin rot advancing that quickly unless the fish is in severe decline. The fins look torn to me, you might be having an issue with nipping in your tank. Guppies are schooling fish and should be bought in groups of 6+ minimum. Having others to swim with reduces aggression and allows the fish to feel safer as well as to fend off bullies, I know male mollies can be notorious fin nippers.

Can you please tell us what other tank mates you have, as well as listing your water parameters?
Nitrate, Nitrite, Ammonia, Ph, and alkalinity?