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Pygmy Corys dying rapidly


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Im hoping someone with more experience can help here and diagnose whats going on.

I purchased 8 pygmy corydoras a couple of weeks ago. They looked very healthy and energetic. They were in quarantine for a week and a half and treated with melafix, pimafix and one round general cure. Theyve now been in the community tank for a week and have looked very healthy the whole time. I did a 20% water change 2 days ago. Suddenly today i looked in to find 2 of them dead right at the front of the tank. One appeared as if it had just died and still had full color and i couldnt see any signs of infection or sickness. I accounted for all of the other fish at the time and they all appeared energetic and happy. 
This evening i looked in on them again and saw this poor little cory looking bloated and struggling to swim. I attached a video below. I dont see any red spots signs of infection or lethargic behavior. Whatever it is seems to be killing them quickly. 

I am currently treating the tank with erythromycin and general cure because i dont know what else to do. Id like to figure out whats going on so that i can prevent this happening in the future.

Temp: 73*F

PH: 7.8

GH: 100ppm

KH: 70ppm

Ammonia: 0ppm

Nitrite: 0ppm

Nitrate: 10ppm

Sick Pygmy Cory Vid

Other corys


Edited by Sliceofnature
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You water parameters seem fine so the only reasonable explanation I can think of is when CoryDoras are under extreme stress they may release a toxin that is lethal to themselves along with other corydoras. They do this in the wild to detur a predator. If this is not the case, I really have no clue what happend. This could truly be a mystery. 

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