So you're saying this hole appears to be right into his intestines?
What size tank? What else is in it? Water parameters? Are there any sharp decorations? What are you feeding?

 

No sharp decorations, just some driftwood and plants in a 55g tank. He lives alongside a blood parrot, firemouth, 6 tetras and 6 rosy barbs. He eats algae wafers and cucumbers stuck on a spoon.

Just driftwood and plants in my 55g tank. Tank has a blood parrot, firemouth, 6 tetras, and 6 rosy barbs. Water parameters are great with a 50% change of water once a week. He's fed algae wafer discs and cucumbers on a spoon.

 

Thiiiis is something not-good. I would suggest you remove him from the tank on the off chance the others will catch it. This looks almost like some kind of skin-rot, like frogs can get... treat with EXTREMELY clean water and the most nutritious food you can possibly get. Some kind of antifungal medication might be useful, though I'd be cautious in case it irritates those wounds.

 

Internal parasite?!

I have no clue otherwise. I would love to know how this happened.

 

found this


http://www.aquariacentral.com/forums/showthread.php?243848-Pleco-Stomach-Rotten-(/page5

 

Doctor, how is the patient?

 

Oh my god that's scary. Do tell us how the fish is doing and what you've done to treat it. I've never seen something like that before.

 

I think you need to stitch it, I don't see how that wound would even heal by itself. Keep the water pristine. It might even start a bacterial infection, it looks pretty big.

 

Phil's link looks to be spot on what you're dealing with. Pull that fish out and drop him in a hospital tank. You'll need to do daily water changes and I'd likely dose both maracyn and maracyn 2 daily following said water changes.

That prior case ended well, which gives hope for this little guy.

 

I wouldn't try to stitch it... it's not bleeding, it would be very difficult to stitch, and it would end up stressing the fish far too much.