Bangaii Cardinal suddenly has problems

Within the 5 min of me watching the tank, my B Cardinal suddenly started having a hard time swimming and got stuck to the overflow. He detached and swam for a min but now is floating around. I just moved him to the QT and he seems to be really ill. What might have caused this? My water params are:
ammonia: 0
nitrite: 0
nitrate: 15-20ppm (working on this with additional refuge with DSB, along with nitrate reducing products like clearmax and instant oceans nitrate reducer)
phosphate: 0
calcium: 400ppm
KH: 9dKH
Salinity: 1.024
temp: 79
Coralife ballast with 10,000k t5 and actinic t5

Ill post pictures of him in a min

 

He just died... :'( He was old, but im not sure what their lifespan is....

These picture are taken after I removed him from the QT.

Any ideas on what could've done him in so fast? He looks free from ich, ect.

 

Now My BTA has folded up into himself... I have never seem my nem do this before... **** I feel like im going to have a complete die off soon... My jawfish has been missing for 2 days as well, he might be dead in there too. I dont understand it. I have been doing weekly water changes and in the past day or so I had a nitrate spike. Going from 0.25 up to almost 20ppm! I can't do another water change either, I ran out of salt and my order is still 2 days away. So frustrated I feel like good ol' Jack D might be comming to visit.

 

Sorry for your troubles. Wish I could be of some help but I can only say test for the obvious like ammonia, nitrites, etc.
I'm assuming you use RO/DI? Carbon block exhausted? Especially if there is chloramine in your water.

 

Yeah I use RO/DI water. I also have 500g of carbon in my sump. Replaced it last week, so it should still be good for at least 2 more weeks. I removed the clearmax I was running as many people don't know much about this product and I think it would just be safer for me to remove it. The BTA started opening a little recently. I googled every different kind of disease there is. I can't find anything that might point out what might be going on. Oh and this is the second cardinal to go within 2 weeks. Before that I had a sally lightfoot, skunk shrimp and a baby hippo tang bite the dust... I figured since all the water params were good that it was my dottyback killing them since I saw him harassing many tank mates. When I removed my dottyback from the DT, I haven't had any deaths until today... maybe he was innocent after all(even though he still kills any hermits he sees)


Also it might help if I list my livestock. Most of you know what affects certain corals/inverts/fish and not others.

Xenia Coral
Lemon Drop BTA
10-15 herms(blue, red and zebra)
10 snails (turbo, nass, astrea)
Green Mandarin
Foxface Rabbitfish
Lemon Damsel
Rainford/jester Goby
sand sifting starfish
4 small brittle starfish
pearly jawfish? (hasent been seen in 48 hours)
Neon Dottyback(in fuge)
percula clown
ocellaris clown
3 pom pom crabs

 

double check you didnt overdose any of the products you are dosing, double check that your test kits hevent expired as well?

Also try testing for copper just to be on the safe side

 

oh and what are you feeding?

 

What maintenance/ changes have you made to the tank recently? Try to retrace your steps and see if you changed anything.

 

What is the size of this tank?

 

That's what I'm wondering. I counted 8 fish & that's not including all that has recently died. But your bta could be closing up cause a recent jump in nitrates.