Panda Cory Barbel Issues

[Reygan2]

It's been 2 months, and 3 of my panda cories have not grown their barbells back. They are in my qt tank, and I keep the parameters pristine. They are eating, active, and seem happy. How long does this take, and what else can I do?

 

[severum mama]

Do you have substrate in the tank? If so, what kind?

 

[Reygan2]

The original tank they were in when I noticed the barbells were gone has pfs substrate. The tank they've been in for at least a month has black sand I bought from petsmart. Sorry I can't think of the name, but it was the pricey kind. It has a finer grain than the pfs.
My params in both tanks are consistantly amm0, trite0, trate<10, ph 7. All my other fish are fine. I haven't lost a fish in months.