Is this diatoms cyano or something else?

[crabinmyshoemouth]

Sorry for the horrible photo but I can’t seem to figure out what this is .Tank is just under a year old .Evwry day my sandbed and rocks gets covered with this brown stuff when my lights come on and it mostly goes away at night . I haven’t tested nitrates in a month but last theme my nitrates were 25 and phosphate I can’t remember exactly but they were on the higherend of what’s considered tolerable. I make my own water using 4 stage rodi and change 20%. A week my tds on my rodi reads .000 . I have no clue what this is or how to stop it ? I just want a clean sandbed and rocks again .

 

[Harold999]

Dinoflagellates probably ostreopsis type, these go into the water column when the lights go out. Running a UV sterilizer is very effective in this case. And run some activated carbon because when this stuff dies it releases toxins.