Acropora tip burn

[Flipside]

You know what else it might be? too much circulation blowing in one continues direction at the coral tips. Are you running some type of wavemaker so you dont have your powerheads blowing so hard continuesly off the tip or your corals.
maybe your temperature fluctuates too much? all these things I mentioned could have a great deal on the matter. maybe all or some combined. or your PH as well. You didnt mention your PH. maybe its too low or too high or it fluctuates between lights being on and off.
Worse case, you should take the burnt tips off to prevent spreading. otherwise you'll loose the whole piece. those are nice collection you got ther.