What's happening on my sandbed?

[DarkReefer]

Hi guys,

Just wondering what might be going on with my sandbed at the moment.
So 2 days after my water change I'm noting quite a bit of an orangish-brownish look to my sand bed, it's done this here and there before but I don't recall it being that quick after a WC.
Last WC was done after 2 weeks (usually done weekly but missed a week, parameters were stable for a couple of weeks prior).

I've also been noticing these bubbles sitting on the sandbed over the last couple of weeks or so.

Can anyone provide any insight to what's going on?

Parameters from today

Salinity - 1.026
Alk - 8.3
Nitrate around 20 (working on this with Nitraphos)
Calcium - 435
Magnesium - 1305
Phos - 0.08ppm

Cheers
Pics taken about 3-4 days after WC.

 

[WVNed]

DIATOMS:

Diatoms are just a natural things that happens in a new aquarium and will normally will subside with in a few weeks to a couple months.


ID: This Algae normally forms on the sand, glass and rock work of a new tank. It has a variety of colors from a dark reddish color to tan, brown and green. On a lot of occasions there will be oxygen bubble's trapped in the algae from photosynthesis.

HOW TO CONTROL IT:
Medium to strong flow across the sand bed and rock, and watch the amounts of Silicate being added to the system ( top off water and water change purity is the main cause for silicate being introduced to the system )

NATURAL HERBIVORES:
Astrea snail, Turbo snail, Tangs, Trochus snail, Almost all blennies.

 

[DarkReefer]

Thanks for that!
Good to know it's just natural process and nothing to be too concerned about.
Maybe I'll have to add some more snails or something that'll eat it.

What about the bubbles though, what causes this?