sudden death

Hi all,

I have an established (12 month)community - angels, barbs, rainbows,
ancistrus, tetras - with only the usual occasional algea problems.

I recently introduced a couple of blue boetia (1/2 the size of the
rainbows, 10 times the size of the tetras) and they seem well behaved
and keep themselves to themsleves.

However, I've lost a couple of otherwise healthy rainbows (separate
occasions) overnight, they're the bigger and more active fish in the
tank but both found floating but with one or both eyes missing, but no
damage otherwise.

I understand that the boetias are more than average carniverous, is it
poss. that the boetia are catching them 'asleep' and killing them? or
is such sudden death to be expected?

Thanks in advance

JF