Guppy fin rot??

Hello everyone? So my guppies tail is not looking too good. I did my weekly water change yesterday and the guppy seemed fine to me. Then today I was shocked to see how awful his tail looks. Water parameters were all good before and after the water change. I have not noticed any fin nipping or bullying in the tank.

It’s a 20 gallon tank with:
- 7 male guppies
- 2 male endlers
- 11 Pygmy corydoras
- Nerite snail
- Amano shrimp
- Neocaridina shrimp

I’m sorry if these aren’t the greatest pics. I can try and get more. Any advice would be very much appreciated!

I should also note post water change I added stress guard and api stress coat