Spiral monti tissue loss overnight.

[BioBoy16]

Woke up this morning to see my monti slimming pretty bad. Didn't think much off it as my two spot blenny loves to just sit right on it. Thought he must have pissed it off. Went for my weekly water change and realized oh crap that's tissue loss. Not sure what happened it was doing great. Don't think it's nudis I've had it for months and the only recent addition was a cats paw I dipped. Any ideas?

 

Parameters:

 

Salinity: 1.025

Ph: holds pretty strong just north of 8

Alkalinity: around 8 but can dip towards the end of the week.

Calc: 415-445 holds strong just small frags in the tank.

Mag: 1253

 

Stocking: 4 blue leg hermits

1 astrea snail

2 nassari snails

peppermint shrimp

tail spot blenny

manadarin goby (spare me ive done the research and its doing just fine. Ive got walls covered in pods atm)

lettuce nudibranch

 

Unfortunately I've been using a FW kit for nitrates and I'm guessing it just doesn't even work. Experiencing cyno and GHA atm. Assuming its getting up there at this point. I use brightwell purit and a small refugium for nitrient export. Cut a big part of the cheato out to spur growth and replace my purit. Only things I dose are red sea ab+ and phyto. One every other day for recommended doeses. Tanks been running since November and only Experienced one coral death bc I acclimated it very very wrong. Learned freshwater dips on leathers are a very bad idea despite what i read lol. I keep up with my water changes. Totally unsure what started this except that maybe the nitrates/phosphates are building up.