Help, what killed my fish

[clr_newbie]

I have never seen the ammonia or nitrite go above 0 in my tank but the nitrate climbed to 10. Why is the nitrate at 10 but I have never seen any presence of nitrite. When will my tank cycle? Here is a detailed summary of the history of my new 30 gal saltwater tank.

Week 1
Set up tank on day 1 with live sand and saltwater purchased from LFS. Tested water daily and noted ammonia, nitrite, and nitrate at 0. Ph was around 8.2

Week 2
Added 10lbs cured live rock and 2 damsels(I read about cycling without live fish after I had done this so please don't chastise me). Tested water daily and noted ammonia at 0, nitrate at 0, nitrate went from 0 to 5 at the end of the week, and PH dropped to 8.0.

Week 3
Added 10lbs cured live rock. Tested water daily and noted no trace of ammonia, nitrite still at 0, nitrate still at 5, PH dropped to 7.8

Week 4
Tested water daily and noted ammonia at 0, nitrite at 0, nitrate rose from 5 to 10 at the end of the week, and PH at 7.8. I lost both damsels at the end of this week. They looked as though they were breathing very hard for 1 1/2 day before they died.

This week
Did a 40 percent water change on Monday. Tested water daily and noted ammonia at 0, nitrite at 0, nitrate at 5, and PH at 8.2

 

[quarryshark]

First of all, welcome to AA

Tested water daily and noted ammonia at 0, nitrite at 0, nitrate rose from 5 to 10 at the end of the week, and PH at 7.8. I lost both damsels at the end of this week. They looked as though they were breathing very hard for 1 1/2 day before they died.

Don't think is was a water quality issue, nitrate at this level would not hurt the fish. PH is a bit low, but don't think it was that either.
I would bet that it was a parasitic outbreak of some sort. With the speed of their death from the onset of symptoms, it may have been velvet. Or maybe even ich. Did you inspect the bodies? Any spots or slimy coating? Were they eating, any scratching (flashing)?