38G Build---First Try

[Magma]

Death has landed in the tank! went to work everything looked fine, come home about 6 hours later and my urchin is in the gravel, no spines left and upside down, about 10 min later (after I got cleaned up from work) theres 4 big bristleworms on him and within another 2 hes hollow...

I checked the water and everything is fine...
Ammonia-0ppm
Nirites-0ppm
Nirates-less than 5ppm
PH-8.0-8.2
SG 1.025
Temp 80

I dont know what I did wrong he was eating like crazy cleaning off a rock I had, half of the rock was clean then all of a sudden he dropped dead...

I did a drip when I first got him, made it last about an hour and changed half the water once the volumed doubled etc. Rather sad thats now 2 inverts I have lost. A peppermint shrimp and a Blue Tuxedo urchin