What to Avoid
While some hermit crab species, like the scarlet variety, tend to be less murderous than others, all will eventually need larger shells to use as homes, and snails are the best target for them. Those same snails are for more efficient at algae and detritus cleanup than the crabs will ever be, so why even bother? Other crabs are often included in crews as well — sally light foots and arrow crabs, in particular, are opportunistic and will often eat whatever they can get their claws on. Even the much beloved emerald crabs (Mithrax sp.) have been known to turn carnivorous for no apparent reason. I would recommend emerald crabs as a last resort in an algae battle, not as a preventative measure.
Ok, so you’ve been converted away from using hermit crabs. In your search for a clean-up crew you come across a crab-less package, which should be perfect, right? The problem is, many crab-less packages contain sand sifting organisms, namely starfish and sea cucumbers. These animals rely on microscopic fauna in the sandbed as their sole food source, and once that food is exhausted the animal most often dies. Unless you have a very large tank, on the order of a couple of square feet of sandbed for each cucumber or starfish, these are best left alone.
Coral banded shrimp and peppermint shrimp are sometimes included as part of the clean-up crew, but often they will turn opportunistic, eating coral polyps, other shrimp and sometimes even small fish. All shrimp need to be well-fed or they will make their own meals, often out of your corals.
While most snail species are harmless algae or detritus eaters, there are a few varieties unsuitable for reef tanks. Murex snails are predatory, feeding on other snails and bivalves. Margarita snails pop up from time to time in shops, but are harvested from temperate locales, making them unsuitable for tropical tanks. Flamingo tongue snails are beautiful and harmless to most motile inverts, but feed solely on gorgonians and so require specialized care. Most conch species are effective algae eaters and scavengers, but grow much too large for most reef systems.
Getting the Crew Together
So what’s the alternative? A snail-only crew is the best way to go. Other than the species mentioned above and a few others, most snail species are small and harmless, unless you happen to be a bit of algae or a scrap of food. Let’s take a look at the specific problems you’ll be trying to address with your clean-up crew, and which species are best suited to solving those problems.
Diatoms on Sand and Rocks:
Diatoms look like abrown dust coating the hard surfaces in your tank. While most diatom blooms run their coarse, they can still be unsightly. Cerith snails and limpets are great choices for eating diatoms.
Green Film Algae:
This is the de facto standard as far as algae goes in reef tanks. It exists in all systems to some degree, and can range from barely noticeable to embarrassing. Astrea snails are amazing film algae eaters (the larger ‘turbo’ varieties are best for larger tanks, as they will knock over small frags in their search for food), along with Ceriths and chitons.
There are a couple types of hair algae. The standard type is non-branching and grows in clumps. Many snails munch on this type of hair algae, including turbo snails and chitons. For larger tanks, non-snail animals like urchins and sea hairs can be the solution to a hair algae problem. The other type of hair algae is Bryopsis. The strands of this type of algae branch off and look like tiny feathers. Most algae-eating animals stay away from this stuff; your best bet here is to solve the issues that are causing the Bryopsis rather than trying to find something that will consume it. Some people have had luck with turbo snails, sea hares and a couple other creatures however. Your results may vary.
This slimy, stringy ‘algae’ can often reach plague proportions in tanks with high nutrient inputs. While the best remedy here is prevention, Ceriths and Nerites are both good choices to help with the cleanup.
If you have a heavy hand when feeding your tank, you’ll want to take advantage of various creatures that will mop up the excess food your fish may miss. Nassarius snails are experts in sifting through the sandbed waiting for a meal, and will pop to the surface at the first whiff of feeding time. For getting into the nooks and crannies of your live rock, it is best to rely on the organisms that hitched in, namely pods and bristleworms. An occasional blast with a turkey baster will also help to keep thinks clean.
Stocking Your Tank
Now that you know which types of organisms to avoid and which to use, let’s go over how you should stock your tank with regards to a clean-up crew. You’ll often see so-called rules saying one snail per gallon; these sorts of rules are far too generic to be of any use, and often result in tanks that are overstocked with snails and crabs. Not all reef systems are created equal, and while one 20g may need fifteen snails, another may not need any.
A much better approach would be more organic. If your tank develops an algae bloom, find out which snails or other organisms have the best track record of handling it, then pick up a few of those. Then, wait a couple weeks to give them a chance to do some work. If the problem does not resolve or gets worse, get a few more and wait another couple weeks.
Most new tanks start off with heavy algae growth that dwindles over time. If you end up with a large clean-up crew toward the beginning, be prepared to whittle it down as time goes on and algae supplies decrease. Mature tanks with effective nutrient export and a low bioload often require very few if any snails to control nuisance algae.
To conclude, hermit crabs do have a place in reef tanks, as interesting additions whose antics never cease to amuse. But for a dedicated clean-up crew, the downsides that hermits bring with them far outweigh their benefits. Look instead for a snail-only crew, and stock your tank according to its needs and not according to an arbitrary estimation based on gallonage.