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Sand Stirrers and Detritivores in Reef Aquaria

by Kenneth Wingerter

These days, a pretty good number of aquarists (a majority of them, perhaps) prefer bare-bottom tanks. Bare, as in, no sand or gravel on the tank floor. Just as these folks opt for bare-bottom display tanks, they opt for bare-bottom refugia. The basis of this trend is a fear of detritus–specifically, that substrates attract and accumulate detritus. Now, detritus does kind of suck, as it looks gross and stores nutrients that can (as they are released) contribute to unwanted algal growth. At least in theory, if you don’t have a substrate, particulate organic matter cannot easily get trapped on the bottom, and so your tank will be cleaner.

This all deserves a closer look. And some deeper thought. 

Sand Stirrers and the limits of mechanical filtration

In bare-bottom tanks, just as in tanks with a substrate, detritus is eventually mineralized (broken down) into excess nutrients and dissolved organic waste. “No problem,” the bare-bottom proponent might say, “I’ll just go heavy on the mechanical filtration.” Fair enough, so long as mechanical filter media are thoroughly rinsed on a regular basis (i.e. daily), very little of entrapped particulate organics can mineralize much. Effective means of nutrient export, right?

Yes and no. Consider first of all that mechanical filtration is indiscriminate. It removes all particles down to some particular size–both bad and good. This means that in addition to all that rinsing, you’ll also have to mess with it (i.e. remove it or somehow divert flow around it) whenever you feed. Otherwise, all of those pricey fish foods and coral foods you add will almost instantly become “waste.” Top-dollar detritus.

But an even bigger concern about aggressive mechanical filtration is pod mortality. Think about it… A coarse mechanical filter medium that cannot strain particles of less than, say, 75 microns is pretty much worthless. Yet, finer media that strain anything smaller than 75 microns will remove most adult copepods and even many copepod nauplii. They’ll be harder yet on larger pods like amphipods. No, we’re not saying that mechanical filtration outright decimates an entire pod population. Nor do we claim that pods simple perish upon first contact with a mechanical filter medium. 

What we are saying is that fine mechanical filtration does trap some (and potentially a lot of) pods and therefore limits their population size. Really “good” mechanical filters can completely eliminate copepod species that undergo a pelagic adult life stage. See the vicious circle here? You keep a bare-bottom tank to enhance the collection of detritus in your mechanical filter, which you install because you have a bare-bottom tank–to the detriment of copepods, which are your best weapon against detritus in the first place!

At this point, we present an alternative (and dare we say more natural?) approach; keeping a substrate, using only super-coarse mechanical filtration (if any) and promoting the degradation of detritus by detritivores such as (but not limited to) copepods. 

The hitch? You need a crew of so-called sand-stirring organisms. Stirrers accomplish two important tasks. Firstly, their excavations encourage the release of organic matter from deep within the substrate. These wastes are otherwise inaccessible to detritivorous animals not just because of layer of sand above them, but also because they may be situated in anoxic areas. This leads to the second job, which is aerating the substrate surface, thereby increasing the depth at which aerobic detritivores can safely forage. 

Substrates=Reef food factories

There is a very large diversity of detritivorous reef critters. As one might expect, many of these sediment-dwellers are tiny. The little guys include microcrustaceans such as harpacticoid copepods, cerith snails, various worms, etc. There are many larger detritivores as well. These range from sand-sifting gobies to sand-sifting sea cucumbers. 

It almost all of these cases, the creatures are not really eating the detritus to eat the detritus; rather, they are going after the rich microfauna that is associated with the detritus (bacteria, fungi, protozoa, etc.). The smallest detritivores just crawl around within the sand and gobble up teeny particles as they find them. Larger ones, however, swallow up big mouthfuls of sand, detritus and all. They then either strain out the goodies and spit the sand back out, or they actually eat the sand, digest the goodies, and then poop the sand. The evidence of this latter group is sometimes observed over the sand bed surface every morning in the form of distinct formed, clean, white sandy pellets.

Not all sand stirrers are detritivores and not all detritivores are sand stirrers. For example, certain fish (e.g. jawfish) and certain shrimp (e.g. pistol shrimp) build elaborate borrows. These burrows help aerate deeper parts of the sand bed through irrigation (that is, movement and exchange of waters between the top and bottom portions of the burrow). This allows other creatures (especially detritivores) to forage deeper in the immediate area, and so compliments and promotes the activities of sand stirrers.

Some creatures help to keep the sand bed clean even without doing any stirring. These range from pods to detritivorous fish (e.g. court jester blennies). The reason is pretty simple: Where detritus is consumed at the substrate surface, less detritus can settle deep within the substrate.  

Basically, having a large and diverse clean-up crew and a diverse team of sand stirrers can significantly reduce the need for mechanical filtration, if not eliminate the need altogether. Some creatures will fill just one of these roles, and others will fill both. What’s important is that the aquarist assembles an ecological community that breaks down detritus as quickly and thoroughly as possible–the widest size particle range, the most types, in all locations of the system. This encourages rapid recycling, which encourages large populations of both zooplankton and bacterioplankton, which are both important components of the coral diet.

Reader Interactions

Comments

  1. ryorzins says

    I would get a diamond goby but my substrate is CaribSea Hawaiian, black. It is not a fine powdery sand. Not sure if the diamond goby can handle sifting my sand.

    Reply

    • dgbsmb says

      I love my Diamond goby, he sifts my sand from one to the other of my 300 gallon reef tank, constantly turning my sand

      Reply

  4. Botzz28 says

    Now that I am populating my tank with eco pods can I keep the more hard to keep starfish other than a regular sand sifting star ?

    Reply

  5. Brian Veach says

    I’ve seen such an improvement in my tank when I focused on keeping the sand clean. We’ve added starfish, crabs, snails, hermits, and sifting fish all with the intention of improving our sand. When we did that, the coral exploded! The benefits have been clear.

    Reply

  6. tonymutti says

    Gobies and snails like Cerith are great. I generally stay away from sand sifting stars because they each a lot of the microfauna.

    Reply

    • Eric says

      I’ve always had some type of ? stirring or sifting creature in my tanks since 2004. My by far favorite is a pistol shrimp with a Goby, no matter the species!

      Reply

      • sfinnell says

        I love my pistol shrimp sharing its home with my goby pair, but the bigger that he grows, the more sand that he constantly moves. He collects it near and far in my tank and as a result disturbs a lot of corals and rock with his never-ending construction. Now, I don’t know if I love him so much! He definitely cleans the sand!!

        Reply

    • Eric says

      I’ve always had some type of ? stirring or sifting creature in my tanks since 2004. My by far favorite is a pistol shrimp with a Goby, no matter the species!

      Reply

    • debken890 says

      Great post. Love my CUC including diamond goby always cleaning sand. Would never go bare bottom doesn’t look natural

      Reply

  21. cmykredesign says

    I use to have a Tiger jaw fish he used little shells to make his house and would throw crabs who got inside his house

    Reply

  43. James Bradshaw says

    I’ve always been a fan of sand so conches are an absolute must for me. Besides being an abnormal creature in my tank that gets oohs and ahhs from guests but they are the best sand stirrers I’ve ever had. With the combination of monthly subscriptions of pods from algae barn, my water changes are every few months max.

    Reply

  63. Jeff Carr says

    Gobies are good, but I have a nano so adding another fish won’t help much. I think I will focus on other CUC types like… pods!

    Reply

  64. Jeff Carr says

    Gobies are good, but I have a nano so adding another fish won’t help much. I think I will focus on other CUC types like… pods!

    Reply

  72. tkshaffer70 says

    Cannot say enough about the yellow-headed sand goby. In my experience they will eat any and every prepared food provided and still spend all day turning over the sand bed. Very attractive as well. Just be aware they will climb up in the water column as expelling the sand. Nice article…very informative!

    Reply

  80. sarah.sandpoint says

    I wonder if I could get some sand sifting gobies when we have a pretty established blenny, he seems okay with all the other fishes – would love to add some gobies, they are pretty cool!

    Reply

  82. blackadonai says

    I already have a diamond goby. He sure moves a lot of sand around. Unfortunately I’m having a cyano outbreak and he won’t touch any of the sand with cyano on it. I also have a sand conch and a tiger tail cucumber that I only see occasionally. I’d also really like to get a couple of those court jester blennies

    Reply

  90. Brandon says

    I’ve always loved having a diamond goby. It’s so fun to watch them gobble sand. Such a cool fish. I need to upgrade my nano so I can have one again.

    Reply

  102. Dan Russell Zabler says

    Never had a diamond goby but I had a watchman goby with a piston shrimp. They always kept my sand nice and white.

    Reply

  103. markwdunlap88 says

    I loved my goby until he became daring and decided to go skydiving during feeding time… didn’t catch him in time. Haven’t tried another yet

    Reply

  106. schmidtcaters says

    One of my constant struggles is keeping enough. And maybe the right balance of sand stirrers in the tank. Keeping pods alive is a huge focus for me as I have a dragonette.

    Reply

  109. Andrea says

    My diamond goby remodels my tank allll the time! Also, if I put a coral on the sand bed, he often says nope and moves it – usually hidden where it is hard to find it. He is a funny little guy, but he keeps my sand very clean…. with lots of mountains.

    Reply

  111. sclarke_o says

    I want a jaw fish but I currently run bare bottom … I’m going to switch I just don’t know when ….. sand stirrers are fun to watch seen them in the wild on a dive!!!

    Reply

    • Austin Clark says

      Looking forward to adding my diamond goby. Just making sure he will have enough copepods to be big and healthy.

      Reply

  128. Kelly Davis says

    I love my pink spotted watchman goby! They don’t usually sift but mine does. Also tries to be an escape artist when given the chance.

    Reply

  163. Noah Fowler says

    The sand is a whole different world I bought a microscope to see all the microscopic live living there. It’s amazing

    Reply

  165. janelle says

    I heard sand sifting starfish can annihilate the organisms in the sand in an incredibly short amount of time

    Reply

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