Faded color on Rummy Nose tetra

Have a question, this evening I noticed that one of my rummy nose tetra's color on it's nose is faded. First time that I have noticed it. The tank has been cycled since the middle of April. Tank is what I would consider to be at least moderatly to heavily planted. Since the tank has cycled I get constant readings of 0 ammonia and nitrites and nitrates have been sitting at a solid 5 ppm. The rummys I added 8 weeks ago to the day. When I first added them to the tank after acclimating them, I did have one that sank to the bottom, after a few seconds it did swim off. Since the group has been added to the tank they have had nice coloration from tbe first day. The group right now is on the small size, 6 I wanted to make sure they did well in my tank before adding a large group and also did not want to overload my tank. Recently I did add 4 angelfish to the tank, this past Sunday. The angels are young and from what I have observed they have been leaving the other fish alone. Last water change was done yesterday, used prime as my water conditioner. On Sunday I had changed out the filter floss in the top of my canister filter. pH is 7.4 and has been since the tank was set up. Temp is at about 78 although temp at times has been up to about 80-82 with this recent heat wave in the area. The rummy that had the faded nose is one of the larger ones in the group, the rest are displaying nice red noses. At this time I am planning on observing the fish to see if anything else looks off. Was wondering if anything stood out that could be a stressor, or any other reason why this fish may be experiencing fading in color, the rest of the coloring on the body snd fins look fine. Thanks.