definitely sick cory - is it catchy to other fish?

One of our peppered cories has really been acting strangely lately. It sits on the bottom of tank or in the 'cave' we made for it in the tank, and stays there for hours. It's coloring never changed, but we did notice it going to the surface for air, so to speak. But our other cory was also doing the surface air gulp, so we weren't too worried. I've noticed them doing this before, so it didn't bother me. Tonight, the sick cory has developed what looks like internal bleeding - is all bloody through the stomach area. None is on the outside though, all under the skin. The second cory is sitting a little bit longer on the gravel, but not as long as the sick one. He swims round the tank, like normal. If this sick cory does die, will get some others for the lone one to 'hang with'. All other fish acting normal. Water change (50%) was done night before last, with good gravel vac.

Tank is a 34g, very understocked with 2 rasboras, 2 white clouds, the 2 cories, and a pleco about 5" long. We havve been waiting for our LF stores to get more of those types of fish to increase the grouping, rather than add other kinds. Sorry - haven't taken the water params yet, will post those tomorrow after work.

Was hoping maybe someone had run into this problem and might recognize it, to start with. He's still alive at this point, but starting to 'porcupine' out around the red area. We've removed him into a fishbowl for the evening to watch him closer, along with his buddy.

Well the sick cory didn't make it through the night, and the other one hasn't developed any other symptoms of anything, so it's back in the tank. Water params are A - 0, Nitrites - 0, Nitrates - 10-15 - says acceptable on booklet, Ph - about 7, GH - normal range - we do have hard water here, could that be part of the problem? (lots of calcium in the water) Other tests all came back as fine, as well. This tank was re-set-up after a bacterial infection decimated the tank 4 months ago. We filled it, cleaned it with salt, cleaned all decor as well, and re-cycled the tank with a single goldfish, before adding anyone else to the tank. Goldfish is fine and in a coldwater tank now, while this one became our tropical tank. None of the other fish are having amy problems.