hap has half a black face suddenly

[Palehorse]

2 days ago my light bar stopped working so i haven't really gotten a good look at my mixed haps since i overhauled the tank on the weekend. I noticed today though, that one side of a female's face has changed from her normal grey/silver colour to black. today i also noticed the heater died sometime in the last few days and that the water temp has dropped about 8 degrees celcius. any ideas on the sudden colour change?
thanks!

 

[mattrox]

Water temp. drop I expect. Fish often go dark when stressed.

 

[JustOneMore20]

I agree. Cichlids especially will get darker or some of them get black lines on their body when they are stressed. Get another heater asap!