Acro

You pH is low at 7.8-8.0 it should be around 8.3, what are your calcium and alkalinity? 

In a salt tank you really need to know the calcium and alk levels they are two of the most important ones. Along with Magnesium.
The acros I got from Chris all died by the next day and polluted my whole tank it really sucked. As Steve said that second one is pretty much a goner:(

I have used baking soda and love it. There are a lot of people the use it to maintain Alk with good success. The dead acro can be just crushed up and put back in, but I would be cautious as mentioned that it could cause some issues if there are dying parts as it could pollute the tank.

You need to add chemicals to your tank to help ensure proper coral health and growth. You can either purchase them pre-formulated in a liquid form or you can mix your own for Alk/Ca/Mg. There are recipes for each of them, and a lot of people use them. If you do not want to add chemicals to feed your corals, you will need to do water changes frequently to keep things stable. The only concern with water changes is that if the salt mix changes, so will your tank parameters. Either way it takes stable water parameters including Alk/Ca/Mg to keep corals happy.

Your thought about just adding in the chunks of limestone to naturally buffer your tank will have some affect, but usually the demand of your corals will far out reach what the limestone will add to the tank. Limestone is used in calcium reactors, but it only desolves in a low pH. That is why CO2 is added to lower the pH in the reactor chamber.

My recommendation would be to get a test kit to know what your Alk/Ca/Mg values are. Then based on the levels of each will determine what you need to supplement your tank with, and how much. Since every tank is different you just need to test and test to make sure things are stable. As an example, I have to add in Alk every day to keep it stable. My 46G tank loses about .5 Alk every day, and it is not heavily stocked. The Ca and Mg usually stay rather stable and only lower slightly, and require less frequent dosing.

I myself would pull it out as when mine died off it really polluted the tank.
You mention only testing alk and cal because you have stony corals but when I had mostly softies I still had to test and dose.

I checked my pH last night and it is a wonderful 8.4.  The 7.8-8.0 that I measured last was right after I had added a bunch of new live mud to my aquariumand was checking that there wasn't anything seriously out of whack with the chemistry while the dust settled.  Always got to look out for the fishies;)

Biodiversity is something I am working on and I am grateful for your frank and forward words. I realize that several of these corals are quite stressed and I am doing my best to keep things stable for them. I have already removed all but a small piece of the dying coral and my tank does have really good surface disturbance with a well ventilated hood and an open sump with a large protein skimmer. The tank is a little more than a year old, but was moved a little over a month ago and was really a fish only tank before now as I didn't have the lighting to support corals until about 10 months ago. I know I have increased my bio diversity a lot over the last month as I have adopted a lot of the life from a friend's dying tank, but is there anything you can suggest? I will try to get a full tank shot up shortly.