I can't understand it either? evrything seems fine with the water unless the PH isn't right it measured 7.8 two days ago but surely this would kill the ray so suddenly?.
The only other things i can think of is the introduction of the new Ray or the changing over from gravel to silica sand? the sand was all cleaned before hand and was changed over correcly as per the guide in one of the forums.
I really want another ray but am very hesitant to put another one in there, the tank looks a bit bare now only got the two parrots and the retic, what other unusual fish would be ok? that arowana of your looks awesome but they grow far too big even for the new 750ltr tank.
Is there a smaller veriant of the arowana?