the rays can get so much larger than the chaca chaca (only get to 7"-8"), and they might eat it (i hear rays like to eat pretty much everything). Also chaca chaca can lower the pH very rapidly, dont know why though. The chaca chaca have a poisonous spine that could hurt the rays if they do try to eat them. I wouldnt want my $15 chaca chaca to kill my $100+ ray because the ray tried to eat it.