lamprologus ocellatus died: full color :S ?

I set up a small tank for keeping shell dwellers, I got male ocellatus
yesterday, the color was absolutely stunning, but soon he'd just lay on
the sand, a couple of hours he wouldn't move is pectoral fins, I came home
a few minutes ago and now I had to take him from the tank because he was
dead, still in full colors. last evening I also put a couple of females
in, yesterday they were swimming throughout the tank, but they too started
getting very colored and they hid themselves in snail shells now, but I
fear they'll die too.

do you have any clues to what has killed him? I suspect some water value,
but I don't know which one, it'd be helpful if I knew because I could act
accordingly then.