Does my tetra have fin rot?

[animal_man1738]

Hello
So about 2 weeks ago I bought 4 tetras from my local fish shop. Two of them look fine, but the third one has some thing going on with his tail. Is it fin rot, or is it some other disease? I have a 55 gallon tank, cycled, soft water, filtered and heated at 80 f. Thanks
PS how do I lower ammonia easily in the tank?

 

[NickAu]

Hi

I cant see a photo