What the #$%^ is going on every time i do a water change.

[zum14]

Ok so Im stumped. I dunno whats going on. I did a water change and a tank clean last night, nothing durastic, cleaned the glass, moved my frogspawn(which is great still) , set a shrimp trap, changed some carbon, moved a powerhead(same flow in area of corals just pointed at sand bed now) and changed a total of about 6 gal of water, its a 46 gallon tank. This is what happend today.....





Im thinking the acro is toast, and the xenia has done that before. My zoas are a bit ****ed and one of my montipora is a little light, but other then that everything else is hunky dory. So i figured it must of been a chemical swing. Tested my dKh. Both are 12.(tested twice with different kits) If the tank was out before i dont think changing 6 gal would bring it back that quick would it? PH is 8.2 on the mix and 8.3 in the tank. Ammonia,Nitrates is zilch on both.I added the water pretty slow with an old powerhead i have so it wasnt turbulent. Im getting to the point where im afraid to do my waterchanges. Any help to get this figured out would be greatly appreciated. OH and im not going to lie the salt mix was prob 2 1/2 weeks old, i keep it in a stock tank in the basement, constantly circulated and heated, i use it for shrimp hatching and waterchanges of course, i just mix 40 or so gal at a time. Is this the problem??? Forgive the stupidity if it is.