Why did my clownfish die?

Okay I have a 10 gallon nano reef with live rock, and some coral that has been cycling for like a year now, It has bristle star fish, and a hermit crab, So I decided to add a clown fish, I properly acclimated him, and about 2 weeks after being added he died. My salinity, and temp were at okay levels, ammonia=0, nitrite =0, nitrate=10. What killed it? Nothing else died, I'd been feeding him brine shrimp, and flake food.