Help: Coraline algae only grows underground?

[chaostactics]

Coraline algae in my tank is plentiful... under my sand bed only. Established coraline on my rocks doesn't seem to shrink but it definitely doesnt grow. I never get coraline algae build up in my glass except for right at the sand bed and about an inch/inch and a half below.



I dont have an crazy clean up crew or anything and I've let the glass go for almost two weeks between cleaning and not a single spot of purple except on the glass that's covered by the sand bed.



What gives?

SETUP

Im 40 gal mostly softie with some LPS that does grow albeit slowly.

Radion gen 4 xr30

IM 40 with upgraded return and MP for flow

BRS 2 part dosing on brs pumps controlled by Apex

Skimmer, bio pellet reactor, carbon reactor, fuge, and purigen

Water changes every 3-4 weeks

 

[Kremis]

do you happen to have any urchins

 

[chaostactics]

do you happen to have any urchins

Nope no other coraline eaters either

 

[hart24601]

I have found coralline loves low light. I would guess the sand is providing shade. The lfs just had a tank sitting cycling while they were trying to decide what to do with it. Ended up being several months and in that time it was totally covered in coralline. The light was only the indirect store light, not a single fixture over that tank.

 

[chaostactics]

The coraline on the left was sub sand level like that on the right until I vacuumed. The tank has been established for at least 6 months maybe longer

 

[hart24601]

The coraline on the left was sub sand level like that on the right until I vacuumed. The tank has been established for at least 6 months maybe longer

I think this makes sense with the lower light. Even if blocking 100% of the tank lights the room lighting is many times perfect for coralline growth and it's slowed if not halted in more light. It's being illuminated from the outside in I think.

 

[chaostactics]

I think this makes sense with the lower light. Even if blocking 100% of the tank lights the room lighting is many times perfect for coralline growth and it's slowed if not halted in more light. It's being illuminated from the outside in I think.

So I've considered this but I'm not getting growth in my rear chambers including the fuge chamber