There a cryptic and twilight zones in my system and have been for years.
Specifically:
a 30 gallon unlighted , uncovered bin with 7inches of sand and a 5 inch layer of live rock on top;
2 44 gallon bins that serve as my sump(s) which also hold some extra live rock and rubble;
a 50 gallon covered bin which serves as an overflow for power
outages and contains about 7inches of rock/rubble and Seachem denitrate matrix media( highly porous pebbles);
A newly added( about 2 months ago) 32 gallon covered Brute garbage can filled to the top with about 150 lbs of live rock. This is plumbed so water enters near the bottom and exits near the top.
I cure/qt new live rock wether it needs it or not for 2 to 3 months in offline bins to insure 0 PO4 and NO3 and to monitor for hitchikers.
The benefits of extra water volume are obvious. I believe the extra surface area is a plus for dentrification , some P04 reduction and reducing organics. Good for pods too. Plenty of sponges and feather dusters .
NO3 and PO4 are low (<0.5ppm NO3/<0.05 ppm PO4) and in my high bioload system.While , the extra rock is not the primary dentirifier in this system which has chaeto refugia ,organic carbon(vodka/vinegar) as well as gfo and gac, I think it helps.
My primary reason for using rock filled cryptic/ twilight zones,however, is diversity particularly sponges given the ability of some sponges to absorb large amounts of dissolved organic material (dom)and spew it off as sponge cells which are believed to be more bioavaialbe to other organisms than the dom. So, the extra rock should, in my opinion help remove potentially harmful organics via pods, sponges, feather duster and other bentic fuana as well as some nitrogen and phosphorous .
Since added the extra 150lbs of live rock I have noticed a few more pods here and there. Beyond that colors and pe on my corals are good and have been. They might be a bit better but I may just be seeing what I want to see. I don't know how you could measure the effect of adding a cryptic zone other than by subjective observations but intuitively it seems right.
Beyond the cost of the rock and the container and extra water , there are no costs to running it for light and flow can be arranged via gravity .
