I can't make sense of this at all, I hope someone else can. Before I begin, here are my parameters: -20G long tank, fully cycled. -5 White Cloud Minnows -2 Cory catfish -All artificial plants/decorations -Currently going through a moderate diatom outbreak So about a month ago I bought 2 Cory's to add to my tank. Two weeks ago I did a 50% water change, which was the first one I'd done since I got them. Up until that point, they both seemed healthy and happy. The next day I couldn't one of them anywhere, but I know they are good hiders, so I figured he'd turn up eventually. Well he did turn up, quite literally, floating dead at the top the next day. So I disposed of him and bought a replacement. Fast forward to yesterday, did another 50% water change (yes, I know 25% weekly is better, I'm working on fitting that into my schedule more efficiently). Once again, both Corys were fine. Changed my water, took the decorations out to clean the diatom growth, and found one of the Corys on his side at the bottom, alive but struggling to breathe. Other guy, no issues at all. After I refilled the tank, I added in Stress coat and stress zyme like I always do and I left everything else alone. I don't have a QT tank, and he wasn't dead, so I let him be in hopes that he'd recover on his own. I figured it was either that, or I'd also find him floating today. Well he's still alive, but still lethargic and breathing heavily. So I'm at a loss here. I don't know if it's just coincidence in the timing, or if the stress of a semi-large water change is to blame, or if there are other factors. The other thing I have no way of knowing is if the current sick Cory is the one that I bought at the same time as my first, or if he is the replacement from the previous dead one, as they both look identical. So I feel like it's possible that if it is the Cory from the first purchase, perhaps it was a bad batch, especially considering the other guy is perfectly fine. So here's my question(s): Does anyone have any theories on what may have happened in either/both cases, is there anything I should do going forward, and is there anything I can do for the sick Cory before he's a goner? Sorry for the novel post guys. I look forward to hearing your responses!