thanks for all the replies.
actually 2 of my Singapore Angel(almost 1 year with me liao) just became not so active and the next few days, they just started to flip and kaput .
The rest just disappeared but ammonia level was 0, PO4 = 0, only thing is Nitrite was up to max 0.1.
That is why I find it a bit strange as all my readings looks ok.
Anyway, thanks again, folks and wishes all a Merry Christmas and a Happy New Year...!
CHeers!