Rusty Holding

[Yanks15]

I noticed at least a week ago that one of my Rusty's is holding. At first she was fairly active in the tank. Then I noticed she retreated to a corner underneath a plant facing head down. She holds that position for quite a while even if other fish come in her area. She doesn't seem to be getting bullied and I do watch the tank quite often. Would this be normal behavior?

Water specs...

NH3- 0
NO2- 0
NO3- 40
pH- 8.0



Thanks,
Steve