Thanks Doug. I agree that the two most likely explanations for what I'm seeing would be light burn and alkalinity burn. If it was someone else's tank those are what I'd be suggesting to look into.

My biggest worry about this whole thing is that, whatever it is, seems to transfer from one tank into another. I have 3 distinct reef systems at the moment and none of them are capable of sustaining acropora species. I do have montipora's, seriatopora's, and pocillipora's that seem OK. In fact a "test frag" of birdsnest I received from Snappy about a year ago has turned into a reasonably sized colony in my 75g. But any acropora's are toast.

This is why I'm personally leaning towards some kind of pathogen as an answer, it's just that I wish there was a better way to confirm and overcome before my new tank becomes the same acropora-free zone that the others are. If it is pathogenic in nature then moving clams and fish and the surviving corals into there are likely transferring the cooties as well.

So, maybe the UV is still the answer. Unfortunately the cost is beyond my means at the moment so the idea will have to wait for now.

Flipside, here is some additional info for you. You're right I forgot to post pH. Here is that info. I was asked the same question on RC so I went and did some testing (I rarely test pH since I've never seen a reason to - it's usually a parameter that is in response to other parameters - depressed pH might indicate high NO3 for example and/or low alkalinity - so I focus on those parameters instead). But anyhow, this is what I found:
pH at end of day before lights out = 8.4
pH in morning two hours before lights on = 8.1

I found that when I was running a reactor, these numbers would be pulled down somewhat (ie. range would be more like 8.2 to 7.8 or so). Right not I am not running a reactor, I dose Ca and Alk manually. Those are the only additives I use. I dose Ca in the morning (Seachem Reef Advantage) and Alk at night (Seachem Reef Builder), I test Ca and Alk every few days and adjust the dosage amounts accordingly if the numbers drift away from target. For the most part it's a steady state though.

The tank in question is not currently running wavemakers. Although FWIW, it did have Tunzes on a multicontroller in the past and that did not seem to make a difference at the time, also my other tanks do run wavemakers. Nevertheless maybe I will see if I can change things around to create less of a steady state flow situation in this cube tank. I don't disagree that this could be a contributing factor, thanks for the suggestions.

I will check my daytime and nighttime temperatures and see if there's a fluctuation at the moment. I don't think there is a significant fluctuation but it's been a while since I checked daytime versus nighttime. I try to run the tank around 76F.

Mabey try dosing smaller amounts multiple times per day. Perhaps you are shocking the system too much with just the 1 large dose. I think that most Alk suppliments have a large affect on local pH.