IMHO rubble in the rear chambers on a nano only serves as a detritus collector. As with any mechanical filtration, all the small particles passing through just get trapped in the rubble and slowly decay. The problem with rubble is the difficulty in keeping it clean. Filter floss can be tossed, but cleaning rubble is awfully hard, unless someone knows a trick I've never heard of.
I also question the use of rubble for its biological properties. The amount of rubble, relative to the amount of LR in the main chamber, really makes me question its utility. I imagine that filling a large volume in the rear with 20 lb in the display and 5 lb in the back does amount to a significant increase in total rock used, but even 20 lb of LR in a 24g tank should provide sufficient surface area for bacterial colonization to handle waste reduction, while still allowing for better water flow control through it, and an easier surface area to clean unlike rubble. I think its a wash as far as biological waste management as whatever increase in surface colonization for bacteria is well offset by the entrapment of crud, which produces the breakdown waste we want out to begin with. As such, I don't see how rubble is any different from bio balls. Both increase the surface area for nitrifying bacteria, and both trap detritus resulting in increased nitrate formation. While LR is supposed to have more "nooks and crannies" for hypoxic/anoxic environments for denitrifying bacteria, the inability to adequately clean the rubble of junk may trap more biological waste requiring processing than any denitrification process may cycle, defeating the very need for nutrient export from a tank, rather than just nutrient processing. A macro like chaeto is great for nitrogen and phosphorus trapping and export by trimming, and can work well in the rear chambers, especially when lit on a reverse cycle to control swings in pH, but to me, rubble is more trouble than it's worth.
Finally, I think that the idea of creating a "refugium" like environment in the rear chambers becomes an issue, as the flow rate through the chambers is awfully high in comparison to a true refugium. Other than the ability to provide reverse lighting cycles in the rear for use with a macroalgae like chaeto, the rear chambers do not have a slow enough flow rate to allow for proper biological filtering, as well as optimal development of food sources for the display tank's inhabitants. Placing the same macro in the display will give you the same nutrient export as in the rear, although, granted, it may be harder to care for.
The light goes on at night. I bought the rubble & chaeto off ebay, but you should be able to get it from your LFS. Rubble is just small pieces of live rock. I wouldnt use filter floss, as I believe thats just a nitrate factory. If you have any questions, let me know!
