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First off welcome.
Second, there is some very good advice already being give in the above posts.
When we had our first tank, before we discovered a good local store, we did buy distilled water from the shop rite because we were only changing out a few gallons at a time. This works, but is...
4 bulb over a 75g might be stretching for a clam, you might have to put it up higher. I would probably go 2x blue plus and 2x coral plus but that is just me.
Anything that is pure skeleton you can toss. The other ones might have a shot. Keep the diatoms off them and acclimate them back to the light. You can try fragging off the living parts but that could also stress and kill them.
I am not a plumbing guy so I will let someone chime in on that.
Looking at your stock list as far as corals go, you might be able to get away without a ton of extra powerheads as you are not really looking at SPS. I don't know what your return pump pushes, but considering you don't have a ton...
People say the accepted range of alk is 7-12 but that is not 100% accurate. 8-9 is really the sweet spot unless you are running something differently, i.e. zeovit users generally run around 7.
Low oxygen can lead to drops in PH, hence with the closed door it may have led to the drop.
Measure your phosphates. If you bring them down to quickly you can shock your corals. That is why you go with half and then increase it. You can test the tank and you can test the water coming out of the reactor as well to give you an idea of how much phosphates the reactor is pulling out.
+1 From Manasquan they are less than a half hour away and they are across the street from each other. Aqua Oceans is south in the other direction, which is a good store but from where you are and hitting two stores, I would go there.
In your current system you are pretty much always going to battle ich due to stress. Queen angels get really big and are really nasty. The powder blue is more likely to come down with ich and having the queen in there is going to make the situation worse. Both of those fish need tanks 180g...