Oto's With Red Mouths?

[mwood2]

Well I'm just kinda curious about this. In my planted 20G tank with full Co2 injection I currently have 5 Oto's inside. Its been around a month since I introduced them and they all seem to be doing well, everyone had nice round full tummies. I have been able to get a good look at their mouths and they seem to be fairly red, is this common for otos? None seem to be sick at all do I assume they must just look like this???

Meant to post this in Tropical Discussion section...