First, my a lot of my zoas have been doing poorly.  The colonies have drastically shrunk over time.  Each time I use the turkey baster to stir up detritus, a few heads come loose and float around the tank. The heads are intact and if a big enough chunk comes off I can glue it to a frag plug, so they are coming detached from the rock. I haven't noticed any sort of of pests, at least that I can see.

Second, I have been dealing with cyano that I just can't shake. I just got some chemiclean to try; online both BRS and Tidal Gardens recommended that or a similar product and both said it produces great results. I just wanted to save it for a last option.

Third, my invertebrates have been suffering. I lost my urchin, and my BTA has looked close to death for a while. I have had a massive snail die off.  I just did a large water change and put some more snails in, and they started looking like they were dying within a few hours, especially the turbos. So my theory of any fluctuations in my alk, which is an ongoing struggle, is out. There is some THING of death in my water.

Fourth, all my macro died off. It may be my light was getting old, which I have since replaced, but haven't got new macro in yet to see.

Overall my fish seem to be doing fine.

Stats: 

3 year old 150 gal tank with 25 gal sump

Temp: between 77-78 with chiller

SG: 1.026

Mg 1200..working on bringing this up..usually it has been fine, my doser ran out for a few days

Ca 450

Alk 8.4

pH 8

No ammonia or nitrite; nitrate ~10

Phosphate 0.05

I haven't used any meds or anything new but I tested for copper which was zero.

I dose weekly with coral vite and amino acids.

No changes in feeding regimen. Add garlic and selcon.

I have plenty of flow.

Interventions:

Just did a 50gal (30%) water change, I use Instant Ocean reef crystals salt; usually I do 30gal every 2-4 weeks

Replaced all filters on my RODI system, TDS is good

Gradually replaced all my T5 bulbs (8)

Running activated carbon in a reactor, skimmer, and UV sterilizer (newish bulb)

Nitrate reducer media pad in sump

Added filter sock

I did have a phosphate spike when the cyano started, for which I added phosguard til it came down. I have done this several times without issues.

Any other ideas?? Anything I've missed? Any input would be appreciated!!

One more suggestion would be to get an ICP test done. They are not too expensive (Not more than a bucket of salt) and it could tell you if you have something rusting or strange in your tank. I don't used the triton method but I could see where a lab test could be helpful in at least eliminating some of the possible culprits. 

1) Immediate 50% water change.

2) Bring Mg up to 1400 immediately afterwards.

3) Do you run carbon? If not, then start. And IF you are running now / or just starting to run - change it weekly for 1 month. Then monthly afterwards. And make sure you are running enough of it. I would suggest a carbon reactor if you don't have one already

4) Do you run GFO? If not, follow same advice as for carbon EXCEPT start with a small amount (1 tsp / 20 gal) and change every 2 weeks.

Equipment wise: 

I do run carbon via a reactor, which I try to change with water changes. I will start to add some GFO to that. I've been watching for for someone to sell another reactor.

I do have a coral life UV sterilizer, it looks fine, no rust.

Skimmer is a reef octopus, not sure of size. I think it is a little small but the previous owner said they ran it on their 150gal with no problems, and it was the only skimmer I could find at the time my old one broke so I went with it. 

4 Circulation pumps are the red sea ones that came with system, each 570gph. I replaced the sump return with a Simplicity 2100gph pump.