the flag may suck a little bit but not much damage
if the flag sucks a big hole in the ray it must be sick in the 1st place to let a ray suck it to death
if your tank is big enough to get out of the flags way it should be fine
my flag only suck on rays when they are preg he seams to get attracted to the movement of the preg lump
you can always train the flag like i did if you see the flag sucking the ray hit it with a stick or tap on the tank glass the flag will learn if he sucks something bad will happen to him
Like most people have said it's 50/50.. I had ome with my rays for months with no problems and then all of a sudden my flagtail wouldn't leave my marble alone.. So needless to say the flagtail had to go..