Help mysterious cory death

[Sky042]

I Bought a panda cory the other day. Actually 4 of them. tonight one all the sudden seemed to give up the ghost. it was laying on the bottom of the tank and I noticed a red blotch seemed to be internal to its central tail region. So I quarantined him off. Then I noticed about an hour later what looked like blood inside near his gils and brain region but it was internal to the fish. he went from swimming around to dead in about 3 hours. What could have caused this?

PH 7.2
NH3/4 0
NO2 0
NO3 ~60ppm
temp 79f

could this be contagious.
Also could he have simply had to work too hard to get around in the tank? I've got kinda strong currents in this tank(result of fluval 304 and 2 301 powerheads)
The cory was about 3/4 - 1" long

Should I be worried?

 

[Tammy2080]

I would Quarantine the rest of the new corrys and watch for the same symptoms.
it could of been shock or an internal bacterial infection both wont be contagious but it is best to Quarantine the fish just incase.

 

[J_acon]

Your NO3 seems high, how long since the last water change? Either cas i dun think NO3 could have killed him. But they are still high.

Look for sign of illness on the other fish, is likely they were sick when they sold to you.

 

[Sky042]

Your NO3 seems high, how long since the last water change? Either cas i dun think NO3 could have killed him. But they are still high.

Look for sign of illness on the other fish, is likely they were sick when they sold to you.

the nitrate is a byproduct of doing the fishless cycle.
it was near 300ppm till I did the major water change prior to adding fish and have since done 2 15% water changes. It's going to take some time to get it really low since my water containts 20ppm from the tap.