Hi Justin
I had similar issues as you that started in Sept 2016 in a 135 gallon SPS system. All my montipora died in short order, all my coralline algae died, many of my invertebrates died (shrimp, hermits, snails) and I lost about 50 % of my acros. The remaining acros lost their color and PE. My alk consumption went from 130 ml a day of alk to about 40 ml a day I tried just about everything, looking for faulty pumps, leaky heaters, rust etc. over a 4 month period.
(A triton test indicated I had a small amount of tin and Aluminum in my system, less than 10 of each)
I ended up doing two things which "seemed" to fix my issue almost overnight.
#1 I looked at the water valve where my RODI was hooked up (cold water valve that also feeds my washing machine). I unscrewed the valve from my home's plumbing and closely inspected it. I noticed that it had been corroding inside. I replaced the valve with a higher quality valve and reattached my RODI. This was the 2nd time this valve had corroded and been replaced.
# 2 I looked for rust again. My search ended with my two little fishies Veggie Mag. I pulled the plastic backing off the in tank half of the Veggie mag, and found the magnet inside to be very corroded. I removed the veggie mag from the system.
After these discoveries I replaced all my stored RODI and did weekly 20 gallon water changes (135 gallon system) I dosed with pantha lith a few times to the tank, about 6 hours before the water changes.
Within a week everything was looking much better, alk consumption steadily increased and my acros' color improved with better with PE.
Funny thing, after I had calculated that I had replaced my water volume with weekly 20 gallon water changes (7 weeks worth), I sent in another triton test. The results indicated that I still had the same amount of tin/Al in the system.
Hopefully this helps in your search for a cause.
Jason