Blackened rainbowfish, is this a disease?

[HenryC]

• pH - 7.6-7.8
• Nitrates - 40ppm (water change day)
• Hardness - liquid rock, about 400 ppm.
• Nitrite - 0
• Ammonia - 0
• KH/Buffer - about 300ppm
• Water Temperature - 75f-79f

Woke up to my rainbowfish being extra energetic and having heavy blackening on its body. He eats ok but I had never seen this so I am worried. Any idea why? The only change I've done is that I added a lot of new plants.

 

[Cory]

Might just be darkening to show dominance.  All the other fish look normal? Is there an air stone in the tank? He's breathing a bit fast and you have lots of wonderful plants. Could be low oxygen in the morning if no air stone. 

[HenryC]

1 hour ago, Cory said:

Might just be darkening to show dominance.  All the other fish look normal? Is there an air stone in the tank? He's breathing a bit fast and you have lots of wonderful plants. Could be low oxygen in the morning if no air stone. 

Hi Cory, he's back to normal now, and all other fish are great. But no, I did not have an airstone (had to move it to make a temporary 10g tank). Just added it back though, just to be sure!