I have had the cories for about 6 weeks, the pleco about 5 months so they are young.

This past week I did my water change as normal on Wednesday, my regular 50% water change. I added in all my ferts except nitrate since I already have enough nitrate. I did not add the calcium chloride and epsom salts that I normally do add to keep the GH around 30 for the plants, I forgot. I did dechlorinate the water and it was the same temperature roughly. I didn't do anything else any different from normal. I also changed the solution in my two co2 bottles and co2 production went up quite a bit from what it previously was. My drop checker was green.

The next morning I noticed that my pleco wasn't acting quite normal, he was sitting in somewhat odd positions in the tank and wasn't active. I added some algae wafers and he showed minimal interest which was unusual. Later on he wouldn't move and when I would touch him with the net he would swim but couldn't seem to maintain buoyancy properly. He didn't swim around much though for me to really see what was going on, but I knew he was sick. I did an immediate 25% water change, thinking that something must have gone wrong with the previous one. I also added in the epsom salts and calcium chloride. He died later that day. The rest of the fish seemed unaffected.

Then yesterday (this being 5 days after the original water change) I noticed that one of my cories wasn't very active either. Then I found him burrowed under some rocks. He also couldn't swim upright and I could see some blood inside one of his barbels like there was internal bleeding. I removed him and put him in a hospital tank and treated him with Jungle Fungus clear (which has antibiotics in it). He died today. After death I saw that both of his barbels had blood showing on them, and there was nothing in the hospital tank for him to have gotten hurt on so it must be a symptom of whatever is wrong. The rest of the fish still appear fine including my other cory.

So to me it seems reasonable that it was not something with the water change, that it was coincidence. If it was, you would see symptoms in more than one fish, happening at the same time, not a fish starting symptoms 5 days after the water change. Also, I don't think it's particularly something environmental, because only one cory got sick; if it were an environmental stress I would think that both fish of the same type would be exhibiting symptoms even if no other fish were affected. Ammonia, nitrite, are 0 and I haven't done anything out of the ordinary with the filter or any other drastic changes to the tank.

So if I eliminate the water change and some environmental factor, I'm left with disease. Not that those other things couldn't play into it, but the root factor must be some type of disease to fit both cases. At this point I'm just hoping I won't see any other symptoms and it will go away...

The only other thing that I can think of, and it's a long shot, is that I changed my co2 bottles. But the co2 level can't be near enough to cause co2 poisoning, at least I wouldn't think so. And I would think that it would affect all fish, not just one here and there. And I didn't think that co2 poisoning would cause swim bladder problems either. My drop checker has been fading from bluish green to lighter green but has definitely never gotten to yellow.