Cories died suddenly - no symptoms

[Todd2]

When I got home from school today I found 2 of my 4 cories on there sides motionless. Sure enough they were dead. They had all been eating and schooling happily together in the previous days. First a little background.

I got these a week ago from Petsmart I wanted cories but I should've waited a couple more days to go to a better LFS. 1 of the cories was really pale, almost looked like an albino cory. The other one was lethargic and generally didn't move. They were in a QT for 2 days (should've been longer ) In the 2 days that they were in there the one that was pale regained all of his colors and his spots were vibrant and he was looking normal. The other one became more active. I then moved them into my main tank (20g L) where they were doing fine, until they died. One of the two dead cories didn't have any barbels. Now, I have sand in my tank so I thought this was strange. Can't barbel loss also be caused by malnutrition or some vitamin lacking in their diet?

Water params. are Ammonia 0 ppm , Nitrite 0 ppm , pH 7.7
I just did a 25 % WC

BTW, right now one of the 2 cories left is eating some food while the other one is under a piece of driftwood (hes been there for a while) and it looks like he's having trouble breathing. Anyone know whats wrong or what I'm doing wrong?

 

[rich311k]

I dont think you did anything wrong. Except maybe not leaving them in QT. I think they just were not healthy fish.

 

[Toirtis]

What species were they (more specifically than 'spotted')? Water temp and nitrates? It sounds like they may have come to you already poorly, and it simply took time for them to show it...however, it is best to leave no possibility uninvestigated.

 

[Todd2]

Water temp: 79 F , Nitrates 10 ppm . The species is corydoras julii (julii cory). The thing is I don't understand why they would 'get better' from the condition they were in at the store and then die. I just read that barbel loss can be caused by poor water quality. The thing is all the cories had their barbels a couple days ago. I guess I need to do a little more research on where I get my fish from. Getting healthy fish in the begining is vital to there survival.

 

[Zagz]

I had a problem with green cories from a lfs here, no matter how many replacements I got from the store they kept mysteriously dying. I switched to albino cories, same lfs different tank, no problems. Sounds like the fish were sick when they got to you.

 

[Toirtis]

Internal problems (like parasites) can often seem to get better for a short while before getting significantly worse...I suspect that was the problem here.

 

[Bubble_B0y]

cories dying suddenly is NOT uncommon.
In fact, there are many articles (just search the net) for unexplained cory deaths.
99% of the deaths occurring with newly bought cories (dying within a week or two of purchase).

As someone said above, the cory may have been sick (and be symptom-free).
I was once bought 6 peppered cories - they all died within a month
My water was PERFECT - the only explanation was that they were sick when I bought them (but showed no symptoms).

I did notice however (from reading the articles on the internet), unexplained sudden cory deaths usually occur in the more sensitive/less hardy cories eg. julii, peppered, panda

For this reason, i'm very hesistant with buying these species.
And ONLY buy bronze/albino cories. They are the MOST hardy cory species !!!