Help! Female Guppy’s tail is turning black!

Hey all, I’m new to fish/aquarium’s, I have a 5 gallon aquarium, 3 female guppy’s, 1 male guppy and a snail. Water was cloudy for a week, and it started getting better the second week, but I noticed one of my females guppy’s tails had turned black. Did a full tank clean, and in the secondary tank her tail started turning less black…. Let water sit in tank for 24 hours, water had been conditioned, added new bubbler and a live plant. Her tail keeps going from Dark Black to more normal and back again? Is she sick? Stress? Help?

This can be a mood thing...

It doesn't look damaged from the photo you've uploaded, did you cycle the tank before getting the guppies? Also do you have a larger tank you can put them in, they will breed like crazy soon!

Nigrocaudatus (dark based body color) female guppies mostly have dark fins. The intensity of the dark fins differ per individual specimen. But when they have mood swings the intensity in the color of the fins can change.