60 Gallon Tank Friend's cories are inexplicably dying

[garlicrbrian]

I'm making this post on the behalf of my friend since he doesn't have a FL account right now.

He has a 60 gallon planted tank he set up in March that is completely cycled. This is the stocking:
12 longfin panda cories
8 guppies
4 boeseman's rainbowfish
2 turquoise rainbowfish (he is planning to get more rainbows to add to the school soon, he just needs to set up his QT tank first)
2 vampire shrimp, 3 amano shrimp
1 otocinclus, 1 farlowella cat
And some rabbit snails

Tank is at 76° F, params are Ammonia 0, Nitrite 0 and Nitrate 5.

The issue is with the cories. He started out with 5 of them, which were perfectly healthy, and about a week later got 10 more to add to the group. They have all been acting completely healthy, eating well and there is no visible sign of illness. However he has inexplicably lost 3 through the past couple of weeks and can't figure out why. Does anybody know what could be causing these deaths, or is it just one of those "sometimes fish die for no reason" situations? All of the other fish are unaffected and doing well.

I can answer any questions you have for him. Thanks!

 

[BigManAquatics]

I had an issue like that with my cories at first, but i think my tank was a bit overstocked at the time for the bacteria level. I have done that before-accidentally given the filter too much bioload for it to handle too soon. So your friend may have something like that going on.

 

[garlicrbrian]

Right. I would definitely consider that but seeing how the nitrites and ammonia are at zero it seems unlikely. He used filter media from an already established tank to kickstart the cycle too

 

[garlicrbrian]

UPDATE - not sure if anybody cares but I thought it was interesting!

J (my friend) deduced what the problem most likely is after he lost three more cories. Since his tank is fairly new and he hasn't gravel cleaned yet, his sand was building up an excess of toxic fermented nitrogen bubbles, which were consumed by the cories, causing them to die. Apparently this is a pretty common death for corydoras and other sand-dwelling fish and animals, which I didn't know about.
He cleaned the sand to release the bubbles which in turn caused his nitrates and nitrites to spike. He has since done a huge water change to get the levels down and we're hoping it will stop his cories from dying.
So, a reminder to those who are unaware - if you have new tanks with undisturbed sand substrate and have/plan to get corydoras or other bottom dwellers, agitate it and get the nitrogen out! It will save your fish!