The difference between Brochis splendens and corydoradine species is that Brochis have more than 10 rays in the dorsal fin while Corydoras have 7 rays. This fish has 7 rays in his dorsal fin like a Corydoras.
I held up a ruler and he's exactly 4 inches long.
Ha! Sorry, I didn't know how catfish-savvy you were. I worked in the aquarium business for, oh, about 15 years and whenever someone inexperienced came in with a "huge green cory" it usually ended up being a Brochis. I've always had a fondness for the cats and currently have a shoal of 12 Corydoras robineae in my 150 gal paludarium.
Since yours sounds like it is definitely a cory, may I ask how are you measuring it? Standard length is measuring from the tip of the nose to the
base of the tail - overall length is nose to
tip of tail. If your cory is 4 inches overall, then it could well be a very well-grown female Corydoras aeneus (Bronze Cory, sometimes called the Green Cory). As you know, females are significantly longer and stouter than males, though yours would certainly be at the top end of those I've seen (they are usually listed as max 3"
standard length, not counting the tail). You often find similar species in fish stores (melantotaenia, for example, also sold as "Green Corys"), but all of these are smaller than the true aeneus. And then there are other corys in the aeneus species complex, which ichthyologists are still trying to straighten out, such as the "Laser Corys" from Peru. I'm not sure what the max size is on those, but I think they are also smaller.
Anyway, post some pics when you have a chance - and definitely separate out that pacu or piranha looking fish until you get the rest stabilized. The tetras were probably too fast for it to bother trying to catch with easier pickings around. Even pacus are pretty nippy (and get enormous, too), the only similar fish I would consider a reasonably safe community fish would be a silver dollar species.
