Cupid Cichlid Has Been Dark Coloured All Day

[STEWARDSONS]

one of my 2 has been dark like in the pic all day and has hardly moved except sit on the sand. next to it is one of the others normal in colour.

any ideas whats wrong with it?

 

[Verminator89]

Its stress, mine did this also. Usually after lights on in the morning it was very dark, and got lighter after a few minutes. Check your water parametres, and if there is any sign of bullying in the tank. Mine was fiesty, but couldnt take the punishment it got back in return

 

[STEWARDSONS]

i have today installed 2 air lines and stones in the tank so maybe thats whats stressed it out. however i do have a rarther aggressive krib in there that in fact bit my hand hard when i was doing it.

whats best for me to do? shall iturn the lights out early? im just worried as its just one, the other two are fine and swimmign around

 

[STEWARDSONS]

just to update, tested water this morning and nitrite was slightly up so have done a 50% water change. i assume thats why the fish is ill.