Acropora tip burn

[Delphinus]

I'm begging for suggestions here. Everytime I try SPS, this is what happens within a couple weeks:









Notice how the damage appears to start at the tips and works down.

Here are the parameters:
SG: 1025
NO3: 2ppm (Elos test kit)
PO4: 0.000 - 0.008 (D-D/Merck test kit)
Alk: 8 (Elos, 2 different test kits)
Ca: 390 (Elos and Salifert)
Mg: 1200 (Elos)
Lighting: 250W DE Ushio - 12 months
Volume turnover: 50x

Tankmates: orchid dottyback, Z. desjardini tang, purple tang, scopas tang, some kind of damsel, 2 doliatus rabbits, a green urchin, a diadema urchin and an abalone. Hallowe'en hermits, blue leg hermits.

So what could do this damage? It seems limited to acroporas right now. There is a porites in there that seems OK. I never see the fish go at the SPS, and the damage doesn't look or sound right for AEFW.

So what's left? The crabs maybe?? There may still be hitchhiker crabs in the tank as I sometimes see their molts. What could do this kind of damage? Look how systemic the damage appears too. It's not localized. I would have guessed alkalinity or lighting but neither of those seem like plausible explanations.

I'm at my wits end with this. I went 6 months without a single SPS in this tank and finally I succumbed to temptation and this is what I get after 2-3 weeks with the new corals. Every single one of them is letting go!!! What gives??