Imminent Tank Crash?

[Zakary2003]

My clean up crew is dying in my 20 cube AIO softie/anemone tank. All but two of my hermits have disapeared and two snails have died in the last 24 hours, one turbo and one astrea. Salinity is 35ppt. Phosphates are elevated to 2ppm (and have been there since thanksgiving so it is stable) but other parameters remain as expected for my tank. 0ppm ammonia and Nitrites. Nitrates are 15 ppm which is relatively low for my tank. I do not have calcium, alkalinity, pH, or magnesium tests here but will go to my LFS to test ASAP. My pH dip sticks say it's around 8-8.2. The only thing I've changed was starting Vibrant three weeks ago to eliminate the hair algae that popped up while I was out of town for Thanksgiving. My non CUC livestock seems fine. My two clowns, their anemone, 6 lined wrasse, and yasha goby are doing fine. My skunk shrimp just molted but appears fine and even has tons of eggs now, and I can't see my pistol shrimp anywhere but can still hear him. My leathers, zoas, star polyps, xenia, palythoa, clove polyps, and Duncan corals all look healthy (so no issues with coral). There is a thick cyanobacteria carpet I've been removing daily, as well as some hair algae which is what made me start vibrant. I feed frozen food and pellets, 1/4 cube of mysis and 1/16th tablespoon of pellets per day. I no longer give reef roids due to my elevated phosphates.

What can I do to find the source of the loss and prevent further issues?

I am currently not running carbon or GFO because the instructions for the vibrant say to stop those treatments, but I can add them back immediately if needed. I also just did a 10% water change right before making this post.

 

[Uncle99]

Check for MG going north of 1500ppm.
High MG has been associated with slow/dead snails.