Ember Tetra Death

Lost a fish today, or rather found half of it hanging in a plant in the tank. I originally had  6 ember tetra and 6 CPD. Tried to count them today and I recon I have five of each, but I think the dead fish was an ember tetra. It was very washed out almost grey in colour and a bit furry. No idea how long it had been dead, last night we went out, found it first thing this morning.
I've checked water parameters, 0,0,20.  pH 7.4
But have done a water change, 50%
My question is, I have another ember tetra which has lost its colour, it's currently hiding in the plants, but is there anything I should be doing? Or trying to do?
Not sure what happened to the missing CPD, but not seen it for a long time. Even when I first bought them about a month ago that one stayed hidden and was hardly ever about.
Any advice / comments welcome.

Thanks Sue, that's reassuring. I do remember you saying that before, but at the time it hadn't happened to me yet!