Cories keep dying

[robertiv]

I have a fairly established (1.5+ years) 38g community tank, a few months back tried to introduce cories into it. They just weren’t happy from the beginning, hiding out & not a ton of activity. Most of them (IIRC there were 6 or 7) died within 10 days.

A while later I tried getting more for the 2 survivors to school with, same result. Those 2 have also since died. FWIW I have one survivor from that second batch, who seems to be doing just fine. I want to get him more company but not unless I figure out what the problem might be.

I haven’t been able to find any obvious red flags during water tests. I do 60% weekly changes, all parameters are within the safety zone – occasionally my nitrates get up to around 20ppm if I’m behind schedule but water changes usually keep them lower. Temps average around 76F. Tank is moderately planted, lots of places to shelter, not overstocked, food-wise there are sinking pellets for the bottom feeders. All my other fish are healthy & active, including ones introduced since either cory batch. However last year I did have the same experience with yo yo loaches, all died within 30 days.

Is there anything cories are particularly sensitive to?

 

[CassCats]

What species of cories?

And what is your GH? Some cories cant handle hard water, some more so than others.

 

[mbsqw1d]

Was it obvious what disease or whatever they were dying from? Any symptoms at all? What are the other fish you keep in there?

 

[utahfish]

Corydora as well as loaches are considered scaleless fish and as such are more sensitive to things in water such as medications fertilizers or chemicals.
In a planted tank high levels of ferts can affect corydoras. Some water conditioners are more harsh than others. Heavy metals are cumulative and can have adverse affects. Just depends but those would be my first guesses.

 

[Lajos_Detari]

Check your GH.

 

[Naughts]

What substrate do you have? Sharp sands or rough stones/ gravel damage the barbels and under bellies which causes infections.