Dead Hawaiian Feather Duster questions

[drfu]

So my tank has been up & running for 8 months and on the weekend two things happened.

First my Hawaiian Feather duster was out of his tube, dead i assume and not really sure why.

Second is the following day i do mu usual water tests & 20% water change and noticed a large nitrate spike! I did a series of partial water changes to bring the level back down to 2 ppm and today for curiosity i did a nitrate spike and its up over 10 ppm.

So my questions are this:
1) so i need to dig out the tube the feather duster was in and that is what is causing the spike?
2) since i don't have any other deaths ti my knowledge where else could the spike come from considering i have not changed anything else in the tank?
3) everything else has great color, looks healthy!

Background info:
IM Nuvo 8. in tank fuge w/ chaeto. custom caddy filter basket with cpe, purigen & ff. spin stream & korelia 240 powerheads.
Chemestry: temp 79.5, sal 1.0245, alk 164, calc 484, mag 1280.
Livestock: royal gramma, 2 sexy shrimp, 2 blue legged hermits, 4 nassarius, 3 nerites
Bta & rock flower anemones, softies, lps, zoas,

 

[Calappidae]

I would get the water tested at the lfs firstly as I've had bigger things die and no spike at all. Test kits go bad.

Seems fishy to me, the chaeto would eat all those nitrates.

Do you have any algea problems?

 

[drfu]

I would get the water tested at the lfs firstly as I've had bigger things die and no spike at all. Test kits go bad.

Seems fishy to me, the chaeto would eat all those nitrates.

Do you have any algea problems?

No algae issues but when i test my other tanks all is good otherwise i would agree the kit could be bad.

I did add some Phosphate Out from Eheim but cant see how that would effect nitrates so I'm at a loss as to what it is. Thx for the reply back