There's a hole in my tetra?

[Hehpoof]

There's a hole in one of my green neon tetras along the blue stripe on the left side (pics below). I first noticed a spot about a month ago and it has doubled or tripled in size since then. No other apparent lesions on this fish or any other fish in the tank currently, though around the same time that I noticed a spot on the tetra, the betta fish developed unilateral pop eye that did not resolve on its own; treated successfully with 8 days of erythromycin (last dose ~2 weeks ago).

The sick tetra is acting normal and not ostracized by the rest of the tetras. Everything else in the tank appears healthy and tankmates behave peacefully. Any idea what's wrong with this guy and how I can help him?

Tank stuff:

• Mature 14gal planted tank with one betta, kuhlis, neon tetras, a hillstream loach, amanos, one vampire shrimp, snails
• pH=6.0
• Nitrate=0
• GH=60
• Nitrite=0
• Ammonia=0
• KH=40
• Water temp=80F

Thanks for help and advice!

[Zenzo]

Is the betta in the same tank? Asking to see if this tetra may have also benefited from the EM med treatment. 

[Hehpoof]

Yes, betta is in the same tank -- erythromycin didn't have any apparent effect on the tetra's lesion.

[Colu]

It could have had a ruptured abscess what I would do is Qarantine and treat with kanaplex in food feeding a small amount twice a day for 7 days and aquarium salt 1 table spoon for 3 gallons the salt will aid Gill function and add essential electrolytes 

[Hehpoof]

Thanks for the suggestion -- I'll set up a hospital tank.

Any other ideas of what this could be? It started as a pinpoint defect in the scales -- never appeared three dimensional or mass-like as would be expected for an abscess.

[Colu]

It could be lateral line erosion that more commonly seen in cichlids the recommended treatment for that is metroplex in food