Dirty sandbed

[Northernseacorals]

Only other thing I can think of is perhaps some old media in the sump, or expired/bad test kit.

Another test you can do to determine if it's diatoms is to vacuum all of them off the sand bed you can possibly see, and in the morning test again for nitrates and you should register something, the diatoms consume the nitrates, if you remove them you should be able to detect some nitrates.