Severe Fin Rot in Guppy

[bigballinfish]

Hi guys, I'm new to fish keeping. I've only had a tank since august 20th, so less than a month.

I have 6 guppies in a 5 gallon tank (I know it's overcrowded, not my choice). One of the guppies in the last day has developed what I believe a severe case of fin rot. I have vaccuumed the gravel, done water changes and checked ammonia levels but the reading seems to be good for now. I believe the fin rot is a result from the first 2 weeks I got the tank I let the ammonia skyrocket as I didn't have the instruments to test or fix it nor to clean the gravel. I don't believe theres any concerning aggression in the tank either, I have not observed any fin nipping or too much bullying.

I know you can prescribe some antibiotics to your guppy to fix the fin rot as its a bacterial infection, but what antibiotics or products should I use to ensure my guppy's fin rot doesn't extend? And how do I administer this medicine?

And can my guppy even regenerate his tail or is the damage too extensive? (The newer photo is the first one with more purple lighting and he seems to be swimming fine for now...)

I've been very concerned for his wellbeing. Thanks

 

[Aiken Drum]

Different antibiotics are available in different locations. Here in the europe, canada and australia for instance antibiotic medications isnt available over the counter and would need a vets prescription.