I know this might not apply to your tank, but when we had our salt water tank, doing too many water changes too often, can sometimes be just as harmful as not doing a water change, as you remove all the good stuff along with the bad stuff.

If you are testing the water, maybe testing the water you are filling the tank with also? And make sure you are adding the correct amount of water conditioner to the water and not overdosing it as well.

But, like the others have said, may be a little too much light, and if you can knock back the food a little, see if that helps maybe?