Cory Problem

Hi,
I've had two cories in my tank for about 5 or 6 years and they've always done well. They usually hang around the bottom and occationally make a dart for the surface, but not very often.
Lately, they've been hanging around the surface almost constantly. They don't appear to be sick or anything.

It's a planted tank and the only thing that has changed is that I added a few new plants; some cabomba, and some ludwigia. Ph is 6.8, nitrates are 20 and the other fish seem fine.

They seem to almost stick their heads right out of the water at times as if they were gasping for oxygen, but there's plenty in the tank, I'm sure with it being that heavily planted.

Any ideas whiat might be happening to them? Could it be my substrate?

Thanks for any ideas.

Shirley

It may be anaerobic because I do get a few bubbles when I poke it with a chop stick, but it's been like that for year.

It has me baffled because I moved one of them to my other tank which has only been set up for about 3 months and he is still hanging around at the surface.

Shirley