Endler Seems To Have "faded"

Hi all,
I'm in the process of introducing myself (in the Welcome forum) but thought I'd ask about one of my Endlers who seems to have faded in colour in the meantime.  For the last couple of days he's drifted rather aimlessly around the tank, not mixing with the other fish and sitting around on the bottom quite a lot.  One of his brothers did much the same for a few days but is now up and about again.  Is this sort of behaviour usual?  (Water is 0 NH3, 0 NO2, 5 NO3)

Here he is on the bottom with one of his brothers whizzing by.

Many thanks,
Ken

I have male endlers at the moment, and bred pure endlers a few years ago. This does happen to individual fish from time to time, and usually ends up with the fish fading away completely. Sitting on one place is not normal for male endlers, though it is for the less active females. I've not found a cure. Keep an eye on it for now, and if it looks to be suffering I treat them like I would a dog in severe pain - I put them down.

Unfortunately I didn't have a camera when I had the endlers. I had top yellow swords from a man in Cumbria and red chests from a woman in south Wales. Both were registered keepers on the British endlersrus website (which has long gone). You can find pics of these two on here. I sold the fry quite easily for a couple of years, then everyone who wanted them had bought them and my market dried up. So I sold them all, fry and parents. But I liked them and missed them so last year I bought some more. I did let them have a couple of broods then sold most of them on, just keeping a few males. There are terrible pics of them here