Snails, crabs, and Xenia dying

Hi,

This past Friday I noticed a large bunch of my Xenia looked a little
"wilty". Kind of blue-grey and flopped over. Didn't think too much of it
because sometimes it does that.

Saturday, it looked worse. Took out one of the rocks that it is a
attached to and it smelled terrible. Definately dying. So, I weeded out
all the dying pieces and checked Nitrates and Ammonia. Ammonia was
undetectable and Nitrates were around 5.

Sunday morning, a bunch of astrea snails are on their backs in the sand.
Not dead but kind of stunned and my tree coral looks almost as bad as
the Xenia did. Flipped the snails over, threw some carbon in the sump,
cleaned the skimmer and checked the water again. Nitrates and Ammonia
are fine. Ph was 8.4, alk 3.4, SG 1.026.

Monday morning the Xenia on the other side of the tank is turning a bit
blue. Tree coral still looks crappy. A few snails are dead.

This morning, Xenia and tree coral unchanged. A few more dead snails and
now some blue leg hermits are dead also. Checked the water again.
Ammonia might be a little elevated. Nitrates still hanging around 5.
Other parameters unchanged. Skimmer isn't putting out excessive amounts
but more than normal.

Now I'm getting water ready to do a large change. Approx 50%.

Its a 75 gallon tank, about 70 lbs of live rock. It's been up for a
little over a year now. Nothing new added in the past 3 weeks. But I did
add some new snails and mushrooms last month. The mushrooms look fine,
star polyps fine, two plate corals are also fine. Fish look fine. Not
sure what else to do other than the water change. Any ideas? Suggestions
to help keep everything from going south?

Thanks,

Andy