Yes, this is Loretta's 3 gallon tank that I put the Cories in...I had bought my first Cory about 2 days ago & he's still living & eating well; then I bought his friend (the one obviously that didn't make it) the next day (yesterday). When I brought my first Cory home I cleaned out the entire tank (kept the old filter in). I don't know why the Cory died; it didn't look like anything was wrong with it, minus the fact that it wasn't swimming around a lot and wasn't eating...
My sympathies on your loss...
But in my (very) limited experience a healthy cory (of the more common types, at least) should appear to be a lively, happy and confident little thing.
I'm afraid your poor little guy likely arrived already overly stressed/unwell.
I've bought in the past probably over a dozen pygmy cories from one LFS and I found I could tell by looking in the bag which ones wouldn't make it.
Any that looked very dark and were inactive.
Those that were lighter (normal colour) and active had no problems.
The five that survived were the only ones I'd been able to find (as that store went out of business) until fairly recently, when I lucked out and got the last 3 from a basement-seller who should, I hope, be able to get more in soon, as I'd like at least 7 or 8 per tank.
These three were, and remain, healthy and lively, (in another tank, as their compatriots were in a tank where mycobacterial infection was most probably present in several fish) although their numbers are low, which isn't good.
But all of these survivors were active even in the bag.
A second point is that a too-thorough tank cleaning may reduce nitrifying bacteria on gravel and other objects within the tank, potentially reducing the capacity just as new fish were added.
I don't know what's entailed in '... cleaned out the entire tank (kept the old filter in). ...', but it's possible this might have contributed to conditions weakening an already sick/stressed fish, as the filter bacteria may not yet have had a chance to repopulate the rest of the tank surface areas, if too well-scrubbed, and thought I'd better mention this for future consideration.
Sometimes it seems that everything, including doing your best, can have potential negative consequences, especially in fish-keeping.
But whether or not there was anything that could have been done differently to alter outcomes - and it's easy to second-guess in looking for explanations - I suspect that had the poor little guy not had problems prior to arrival, he'd be alive today.
Especially since the other seems to be doing rather well.
and well, I am not sure if the tank needs to be cleaned out or not?
