I am so frustrated!!

[seanoman]

Over the last few months, we have been battling something with our tank. It is a 220 gallon sps tank. It has now been up for about 15 Months. It seemed to be right around the 1 year mark when everything started going wrong. We had great growth and good colouring on everything. Then all of a sudden it has gone down hill from there. We have lost several colonies and continue to battle to hold on to some right now. Some corals shows no signs what so ever, and then others do. They start to lose flesh, in no particular spot, then eventually completely die. I have checked all my parameters, and they all seem ok to me, except for my alk which might be a little low.

Calcium - 460 (maybe a little high)
Alk - 6 - 6.5 DkH
MG - 1150
PO4 - less than .01
Salinity - 1.026
Nitrate - undetectable
Nitrite - undetectable
Ammonia - undectable

These were all done with Elos test kits aswell.

Every time I do a water change, I do notice an improvement, but within 3-4 days after, it seems to decline. Now I have tried to look up the affects of low DkH, but haven't been able to find much. Is it possible that low Dkh will have this affect on corals? I am trying to slowly raise my Alk with dosing kalkwasser. Any help or advise on this would be greatly appreciated. I am so lost and losing a lot of interest in the hobby. I am not about to give up though, because I know when we figure this out, it will be much better afterwards.

Thanks in advance.