Help, fish death!

[syncro]

I could some some diagnostic help.

The situation:
- Found 1 dead benggai cardinal fish stuck to the MP10 today
- Other cardinals and clownfish are NOT eating
- Introduced 9 cerith snails + 1 hermit last weekend (no quarantine)
- Prior to that, last livestock change was 4 months ago (from quarantine)
- No white spots on surviving fish
- Water params reasonable and stable
- Last weekend, installed an ATS (really just put a white plastic mesh in the sump horizontally and a light bulb)
- Water params stable, using controller for ATO and temp
- ph: 7.7 - 8.1
- Ammonia/nitrite/nitrate: 0/0/0
- salinity: 38ppt (I have been lowering at rate of 1ppt/week)

Have I introduced marine ich or velvet? Other possibilities?

Thanks!