When working with rocks, live or otherwise, in marine aquaria, we often end up with little chunks of rubble that break off the larger pieces. While this rubble may not be particularly useful for large aquascaping projects, it can come in quite handy in a variety of different ways, so don’t be too quick to give it the heave-ho along with those shipping containers your rock order arrived in.
Here are just a few good uses for rock rubble off the top of my head. If you can come up with some other good ones (Paul B, with your innovative mind, I’m sure you’ve got more than a few ideas here!), please feel free to share them in the comment section below this post.
1. Coral attachment sites
Starting with the most obvious possibility, rock rubble of varying sizes can be used as an attachment site for coral frags or detached polyps that you can then place in other areas of your tank or trade with fellow hobbyists/local dealers. Depending on the type of coral you’re propagating, you can either glue/band/tie a frag onto the rubble chunks or place the rubble in a tray, set loose polyps on top of the rubble, and wait for them to attach naturally.
2. Snail substrate bridges
Certain snail species commonly used in reef cleanup crews, e.g., Astraea spp., have a very difficult time righting themselves if they end up lying on their sides or upside down on sand or fine gravel. This sometimes happens if they’re knocked loose by a fish or if they attempt to travel off the rocks and across the fine substrate to reach greener grazing “pastures.” Thus, hobbyists who keep them in tanks with such a substrate must be prepared to retrieve fallen snails and promptly return them to the rockwork where they can get a firm purchase.
I’ve found that one way to reduce the risk of Astraea or similar species winding up stranded on a sand bed is to create little bridges of rubble that extend from the rockwork to the glass at several locations around the tank (e.g., the corners). That way they can get wherever they want to go in the system without touching the substrate.
3. Construction material for burrowing species
Burrowing species, such as the various jawfishes or shrimp gobies and their crustacean symbionts, can’t construct stable subterranean residences if all they have to work with is uniformly fine sand or gravel. Burrows constructed out of such materials are inclined to potentially deadly cave-ins. However, if you mix some rubble into the substrate or simply scatter it on the surface of sand/gravel, these industrious fishes will be better equipped to build sturdy burrows that are resistant to collapse.
4. Quarantine biofilter seeding
Providing adequate biofiltration in quarantine systems, which are often set up just prior to the acquisition of new specimens, is a perennial challenge. One way to help this process along is to keep a good amount of rock rubble somewhere in your display system (e.g., the sump) so it has a chance to become inoculated with nitrifying bacteria and then move some of it to your quarantine tank just before the new specimen is introduced.
I’ll sometimes put a good amount of seeded rubble into the chamber of an old HOB filter and install it on my quarantine tank to provide supplementary biofiltration and enhance water circulation/aeration.
5. Clownfish tailwind control
Anyone who has ever owned a good-sized clownfish—like the bruiser of a tomato clown (Amphiprion frenatus) that currently occupies my system—knows these fishes have a habit of rearranging sand beds via tail-fanning. This behavior can become problematic if it undermines rockwork or freestanding corals/clams or if it causes “sandstorms” that deposit substrate material onto invertebrates situated low in the tank.
To counteract this behavior in my tomato clown—which kept causing a freestanding gorgonian to tip over into the resultant pit—I simply scattered rubble all around this particular gorgonian’s corner of the tank. The clownfish still does its tail-fanning thing, but the “tailwind” no longer disturbs the sand beneath the rubble, so problem solved!