Lost all my 3 week old fry overnight - Pics within

So over the course of 48 hours I lost all but 3 of 80 fry. The remaining 3 are about to be culled to be put out of their misery.

I have taken images of the obvious illness, yet note that I have only noticed these white spots developing today on these fish. The previous ones that died did not display any clear sign of infection or illness.

Attachment 21558

Attachment 21559

My water is changed daily ranging from 50 to 80%.
pH - 7.5
Temp 29
PPM 50 (remineralized RO)

Amm - 0
N02 - 0
N03 - 0 (undetectable on SERA/API kits)


Is this ich, or velvet? Furthermore, could this be the culprit behind the sudden onslaught of all of the fry without any clear symptoms, or is this likely a secondary infection caused by stress of something else that wiped them out?

Recently taken BBS, in multiple, very small amounts.

Any advice would be most appreciated.