Layering with different grain sizes or just fine?

[Orson]

Question:
I've come across two schools of thought:
1. 2 layers of different sand/gravel grain sizes seperated by a gravel tidy (fine mesh). Fine gravel/sand ontop of coarse gravel. Often associated with the use of a plenum. Like the Jaubert system.
2. 1 layer of fine sand. Tends not to include a plenum. Just dump it in there.

My understanding is that different layers of gravel/sand promotes better water movement and subsequently more stability (no sulphur dioxide formation) as sand depth increases.

I've read that exceeeding 10cm in depth increases the likely hood of suphur dioxide being produced. Particularly without a plenum? So isn't sand worse?
Obviously stability and functionality increases as do the worms and grubs in the sand in both types. I've noticed though that the worms prefer the fine sand.

Quite simply why is the 2nd optioin seemingly more popular? Obviously one is easier to construct than the other!

 

[sihaya]

Question:
I've come across two schools of thought:
1. 2 layers of different sand/gravel grain sizes seperated by a gravel tidy (fine mesh). Fine gravel/sand ontop of coarse gravel.

I don't think this arrangement improves waterflow to the fine sand at all (I just don't see how/why it would). The only possibly good idea for a "2 layer" system I've heard of is if you put some rubble on top of fine sand so that you can increase your over-all waterflow without having your fine sand "fly away."

 

[Reef Maniac]

My understanding is that different layers of gravel/sand promotes better water movement and subsequently more stability (no sulphur dioxide formation) as sand depth increases.

WRONG. You WANT the bottom of the layer to become anaerobic (low in oxygen) in order to complete the (nitrogen) filtration cycle and remove the nitrate from the water, and having a "better water movement" defeats the object.

I've read that exceeeding 10cm in depth increases the likely hood of suphur dioxide being produced. Particularly without a plenum? So isn't sand worse?

Why? Don't believe the "old wives tale" that sulfur dioxide is deadly poisonous and will wipe out your tank - if left undisturbed (as a DLSB should be) there is absolutely no problem with "black zones" developing on the bottom of the layer - in fact, the anaerobic bacteria which causes these "black zones" actually also convert nitrate into inert nitrogen, and are thus assisting with your filtration.

Quite simply why is the 2nd optioin seemingly more popular? Obviously one is easier to construct than the other

Yes. Also, a plenum has absolutely no advantage over a DLSB, and tends to go "off" easier than a DLSB. So, why make life more difficult than it already is?

Hennie

 

[Tony]

I tried a plenum once and it never really worked although the idea was to put the crushed coral as the first five cm layer above the void as aragonite clumps and turn to cement especially where an overflow from a baffle hits it. I experienced this clumping first hand