Help with Diatoms in 1 Year Old Tank

Tank Specs:
75gal DT (9 fish, 14 frags)
40gal sump with fuge. (Actual operating volume roughly 30gal)
Sal-1.025
Temp-79
Ammonia-0
Nitrite-0
Nitrate-5
Alk-9.2
Cal-435
Mag-1480
Lights -RapidLED kit. Intensity settings RB-70%, white-50%, UV-50%
Carbon - yes
GFO - yes

Almost a month ago I notice diatoms on the sand bed. I changed my RO/DI filters and resin, which had not been changed since I started the tank a year ago. I'm assuming my RO/DI water was the source of the silicates that caused the diatom bloom. But it has been roughly a month since I changed the filters and the diatoms are still here. I've done several water changes and I've vacuumed the sand bed, but it hasn't helped. It's not on the rocks, just the sand. I'm not feeding any more or less than I usually do, which is typically twice per day. Does it usually take several weeks for diatoms to go away? I can't think of anything in the tank that can be fueling them. Any advice would be greatly appreciated.

 

I'm wondering if it may be cyano and not diatoms by your description......

 

This stuff is a golden brown. I had cyano before and it was reddish and a different consistency. The cyano was cured relatively quickly by adjusting the flow to the dead spots.

 

Can we maybe get a picture of it ? It could also be dino

 

There are no bubbles that you would see in dino. Nor is it stringy like the dino I've seen in picutures.

I'll take a pic tomorrow when the lights are back on.