Has anyone ever seen this type of damage before?

Hmongol

I did remove him, thanks Romad and 1077. I've read this websites info in them, I just can't believe that a Petco employee would recommend this guy to my Guppy and Neon Tetra community.

I am =)

redchigh

Instead of anti-bacterial, I'd use salt to help him heal. (Salt's dangerous, but when it comes to livebearers it's generally a good treatment.)

Be sure to do it in another tank.

Teishokue

I wouldnt trust most petco/petsmart or big animal dept stores. if you are going to ask for a fish specialist most likely you should read up on it is the first thing, the second is to ask at a LFS (no other animals but FISH) that way you know thats all they know for the most part, then ask on forums if you still are questioning. as CAE grow they will get more aggressive and eat less algae. are you sure you have a CAE or a False CAE>?

Hmongol

I'm sure, when I bought him he had a name of just Algae Eater but in parentheses Gyrinocheilus and Gyrinocheilus is also known as a CAE.

Teishokue, are you Japanese?

Teishokue

Kuj yog hmoob thiab os.

Im aware most places like that would sell it as Algae Eaters but what im saying is are you identifying it as a CAE or a False CAE. since they are very similar with small differences. It is just like the False Percula Clownfish (Amphiprion ocellaris) vs the True Percula Clownfish (Amphiprion percula).... the thing you can tell them apart for sure is that the false one has 11 spines on the dorsal fin while the true has 10.. others have observed more but havent been too consistent.
in your case there are the species:

A type of Crossocheilus siamensis species

Epalzeorhynchos kalopterus
Garra taeniata

Or a type of Gyrinocheilus species

Eshnon

Get some aquarium salt. It helps with recovery, gives electrolytes fish need, AND improves gill function. Your Guppy is in a lot of pain right now, but aquarium salt will make it so much easier for your Guppy to survive and heal. I put in aquarium salt anyways just because it promotes good healthy fish.