I think it just depends on the rays and their size. I have lost many fish to my rays. Angelfish (eated like potato chips), then tried larger geophagus (5-8") lasted a month or so but finally got eaten. Tried a fairly large S. daemon (7-8") was eaten. My rays only seem to eat fish when they are pregnent, this has been when virtually all of these fish were lost.
They have not bothered my shoal of clown loaches though (all 7-9"), I think this is because the loaches are active at night where the other fish seem to become less active and more listless during the night time making them easy pickings for the rays.
They have not bothered my shoal of clown loaches though (all 7-9"), I think this is because the loaches are active at night where the other fish seem to become less active and more listless during the night time making them easy pickings for the rays.