I've devoted some time to your situation this morning. I went back and reread every post about your aquariums from your first day posting. Looking at all this information and given that you lost another fish today, I would have to think water is an issue.
I see a couple of things in your posts that make me think pH might be the culprit. First, pH can swing up and down a great degree in a given day, and these swings can be even greater in systems with inadequate alkalinity monitoring and supplementation. Your pH and alkalinity readings appear to be unpredictable and vary greatly from one test to the next. I have to again move this conversation to the testing and supplementation of calcium. I see that we discussed calcium, but I do not see that you have taken any measures yet to test or adjust the levels. As I've stated before, if I could only test for 2 things in my tank, these things would be calcium and alkalinity.
You've requested more details from me, so this will be a long winded post. Let me explain why calcium is so important and can cause problems if not correct. The pH in your aquarium is not a constant. It moves up and down, with the lowest test being in the morning and highest at night. This is a result of CO2 levels in your aquarium, which accumulate at night when the lights go out. This is due to the photosynthesis of algae and/or corals and happens in all aquariums, freshwater and salt.
This swing that your pH experiences will vary. The wider the swing, the greater the stress on livestock. Alkalinity is a measure of the buffering capacity of your water. Carbonate ions, bicarbonate ions, and borate make up the majority of your buffering ability. This buffering capacity will determine the floor of the pH level and control the swings. The most highly utilized buffer in a marine aquarium is calcium; more on this later.
These buffers are depleted from the water by acids, which we call "organics" in this hobby. Organic acids are released by marine fish directly into the water, and result from food and waste decomposing inside the aquarium. These organics are removed directly by a protein skimmer, and are broken down into Nitrates on systems which utilize mechanical and/or biological filtration. Additionally, these systems see phospahte accumulation as a result of this biological process. The more effecient your skimmer, the quicker the organic acids are removed from the system. As a result, the less carbonate ions are depleted, the more stable your alkalinity, and the smaller the pH swing in your aquarium.
Organic acids are also removed biologically by live rock, in a process of breakdown similar to a biological filter. The difference with live rock is that Nitrates are not the end result. Live rock contains additional backteria that convert the Nitrate into Nitrogen Gas, which leaves the system naturally. So, systems with larger amounts of live rock are capable of supporting larger amounts of organic waste, as are systems with larger and more efficient protein skimmers. Live rock and the protein skimmer work hand in hand to eliminate organic acids before they become nitrate and before they put a strain on your carbonate buffer system.
Entire books have been written on this topic, so let me wrap this up. In your aquarium, we have seen the occassional ammonia reading and we have seen swings in the pH reading. You are doing water changes regularly, which is helping to replenish some of the buffers, but water changes alone can not replace the levels of calcium necessay to ensure alkalinity can remain stable and control the pH swings. I believe you are adding a calcium additive, but I do not see that you are testing calcium to know how much to add. Every system is different (for the reasons above), so you can't just follow instructions on the bottle.
The absence of a controlled calcium supplementation program is causing other buffering ions to be utilized at a heightened rate that we normally do not see. Because of this, I believe that the ions which make up your buffer system are very unbalanced. Some ions are being depleted rapidly, which others are non existant. It is these ions which make up the very nature of what we call "saltwater". Water with salt is not "saltwater" by marine aquarium standards unless it contains the correct ions in the correct proportions. This lack of balance is causing great strain on many things in your system, and a great level of stress on your fish.
I suggest increasing the amount of live rock in your 80 gallon tank until you reach 80-120 pounds. Begin testing calcium, alkalinity, and pH daily (at the same time every day) and logging the results.
Here are some references that could help you understand this process further: Advanced Aquarist's Online Magazine - Aquarium Chemistry: The Carbonate System in the Aquarium, and the Ocean, Part III: Methods Available to the Aquarist. Chemistry and the Aquarium Calcium and Alkalinity by Randy Holmes-Farley - Reefkeeping.com
I highly recommend that you devote a couple of days to reading everything you can find by Randy Holmes Farley Randy Holmes-Farley - Reefkeeping Online Magazine
, who is widely consdiered the final expert on all topics releated to chemistry in the marine aquarium.