But first, I need to finish this section and include a little about the unusual filtration needs of n octopus.  Octopuses are almost continually shedding the skin on their suckers and arms, and so there tends to be a lot of large bits floating around that need to be removed from the tank before they begin to rot.  In addition to taking only large prey items, they tend to be somewhat messy eaters, and water quality can be an issue if you do not have adequate filtration in place to keep up with the production of waste generated by an octopus.  The shedding of skin is one of the primary reasons that I like to use a mechanical power filter of some sort on an octopus tank – most such filters on the market today will do a good job of removing and collecting all the floating skin from the tank quite quickly.  However, this also means that regular maintenance is required to prevent these filters from clogging up.  Obviously, if the filter is removing a lot of shed skin from the tank on a continuous basis, that will quickly clog up the pores of any mechanical filter, and need to be cleaned accordingly.  A seriously clogged powerfilter can be more harm than good to an octopus, and you must keep on top of the filter cleaning on a regular basis.  Depending on the size of the tank, and the size of your octopus, this could be as often as a couple of times a week in some cases.  Whatever the size of your tank and your pet octopus, it is always better to err on the side of caution and clean your mechanical filter more often rather than not frequently enough…

An aside about oxygen concentrations in seawater

The amount of oxygen which can dissolve in salt water depends on the exact salinity and temperature of your aquarium -- the table below gives you the values of oxygen saturation (mg/l) across a wide range of temperatures and salinities. Saturation is the point at which no more oxygen can dissolve in the water under normal conditions.  There are conditions under which excess oxygen can be forced to dissolve into the water (such as when water is under pressure in a pump, for example), however, and under such circumstances, the water can become supersaturated with oxygen.  Ignoring the variability across temperatures and salinities not seen naturally on coral reefs, the basic answer is that saturation for an average marine aquarium is roughly 6.5 mg/l.  However, this value means relatively little to the overall health of your animals (which I will explain in detail below) because you may have far less or far more oxygen in the water depending on the specific conditions in your aquarium.

Table 1: Oxygen saturation concentration (mg / L) as a function of salinity and temperature of aquarium water.  

Having said that, it makes sense to ask whether our tanks need to be at saturation, and what normal levels of oxygen are in the ocean in areas around coral reefs.  I probably don’t have to work very hard to convince you that surf zones regularly pounded with waves aerate the water, and are probably always nearly saturated with oxygen.  But what about the areas around coral reefs – how much oxygen is usually found in that water? 

Well, a number of studies have shown that seawater in areas around natural coral reefs are generally between 95-110% saturated with oxygen (i.e., 110% saturated means 10% more oxygen in the water than the value given in the table above).  On average, natural coral reef areas are very close to 100% saturation (e.g., Kuhl et al. 1995; Kraines et al. 1996; Leclercq et al. 1999).  Obviously oxygen is important to our animals, and if water around natural reefs is usually saturated with oxygen, then we should probably strive to maintain oxygen levels close to saturation in our aquaria as well (e.g., Ng et al. 1992; Toonen 2000).

However, there is plenty of variability in the system.  For example, measurements around one Caribbean island showed that night time oxygen levels range from about 5.0-6.5 mg oxygen/l, and daytime levels rise to around 7.5-9.0 mg oxygen/l (saturation was roughly 6.25mg/l) depending on the reef me asured (Adey and Loveland 1998).  Daytime oxygen concentration rise above saturation because photosynthesis produces oxygen at a higher rate than it can leave the water by diffusion.  This leads to the odd situation in which the areas of lowest oxygen concentration on the reef are actually those with the most gas exchange, because in highly turbulent areas with extensive gas exchange, oxygen diffuses out of the water more rapidly and is closer to the saturation level of about 6.25mg/l.  On the other hand, in areas with more limited gas exchange, there are actually higher oxygen concentrations during daylight hours because photosynthesis produces oxygen faster than it can leave, and therefore it accumulates in those areas.  The opposite is true at night, though, when photosynthesis does not occur, and instead respiration uses up available oxygen (Adey and Loveland 1998).  Therefore, the areas with the lowest gas exchange cannot replace the oxygen that is being used up, and the concentration falls the farthest below saturation overnight.  Walter Adey reports similar levels of oxygen in his ATS-based tanks at the Smithsonian, but is also quick to point out that it is not the oxygen concentration itself that is important but rather the exchange rate of oxygen that is important to the health of the an imals (e.g., Adey and Loveland 1998; Leclercq et al. 1999).

Invertebrates use something like 0.2 – 15 μL of oxygen on average per mg of body weight per hour (Bailey et al. 1994), depending on the species and a wide range of factors including size, temperature, activity level, and so on.  So, what exactly does that mean in the real world?  Well, let's say you start with an average well-stocked 50G reef aquarium full of invertebrates and a few fish. The respiration of the animals in the aquarium would likely be on the order of ~ 2-3g of oxygen per hour. Even if you could supersaturate the seawater (lets say it's at 9mg oxygen/l -- the highest recorded on the natural reefs I mentioned), that still only gives you about an hour before the animals would begin to show signs of suffocation if gas exchange is limited.  Of course, protein skimming, turbulent water flow (e.g., "dueling" powerheads, and especially surge devices), and photosynthesis will alter that rate of exchange, and with the normal exchange rates of roughly 4-6g of oxygen per square meter of surface area per hour (e.g., Leclercq et al. 1999), the respiratory needs of your animals should be easily met. While it is true that both salinity and temperature will affect the particular value of the oxygen saturation coefficient in seawater (as outlined above) the actual maximum value of how much oxygen can dissolve in your tank is pretty much a non-factor during normal operation.

 So, the bottom line is whether oxygen can get back into your tank fast enough to prevent it from being used up by the animals in your tank.  In the simplest terms, without sufficient turn-over and current to allow efficient gas exchange, the rate of oxygen replacement into your tank water is not able to keep up with the rate at which is being removed by the animals and bacteria in the tank and filter.  Sadly, many people do not realize that oxygen concentration is low in their aquarium until it is too late for the octopus.  That extreme sensitivity to low oxygen concentration is the main reason that I recommend high turn-over skimmers for any octopus tank – although the skimmer tends to maintain better water quality, and is therefore likely to be healthier for your animal, the highly efficient gas exchange that takes place in the skimmer and maintains a flow of highly oxygenated water into the tank is well worth the cost of a skimmer on any octopus aquarium.  The water coming out of an efficient protein skimmer usually contains something close to ~6 mg/L of oxygen in an average reef aquarium (depending on the exact temperature and salinity of the water – as I explained above).  As long as the rate of oxygen use in the aquarium does not exceed that of replacement via skimming and turbulent mixing in the aquarium, your octopus should be fine.  As soon as that well-oxygenated water enters the aquarium, however, your animals and even the bacteria in your tank and filter begin to use the oxygen within it, and the more animals and bacteria there are in the tank, the faster that oxygen is used up.  So, the smaller the tank, and the larger the bioload, and the lower the flow rate and surface area available for gas exchange, the bigger problem that low oxygen is likely to be in your aquarium.

Keeping your octopus in its tank

As I mentioned above, I generally prefer to use a well-sealed aquarium with an open sump in which to keep the skimmer and filtration, but a carefully sealed tank without a sump can work just as well, provided you have a reasonably large tank (say at least 30 gallons for an octopus whose head is about the size of a mandarin orange) and good filtration.  The primary problem with a sealed tank is that it limits gas exchange with the air outside the tank, and can result in decreased oxygen availability within the aquarium.  There are two general ways to deal with this.  The first, and generally the best option is to have a sealed top that is octopus-proof, but not air tight.  An example of this would be a top made of plastic egg crate (the light diffusion panel sold at many hardware stores) that is covered with shade cloth.  In this way, you have a solid lid that prevents the octopus from escaping, but you still have an open top that allows for efficient gas exchange.  The second option, if it is not possible for you to make a mesh top for some reason, is to have a solid top (either glass or plexiglass) with an external air intake for your filtration.  This can be as simple as an air pump running a couple of airstones in the tank, or making sure that your air intake on your protein skimmer is plumbed in such a way that it draws from the room rather than inside your tank.  By having air from the room continuously added into the aquarium, you should ensure that the oxygen demand of your tank will be met, even if the air exchange at the top of the aquarium is limited by a solid top. 

My recommendation here is obviously not the only way that an octopus can be kept in captivity, but I am offering you my opinion for the way in which you are most likely to have success in keeping an octopus happy and healthy throughout it’s lifespan.  For example, I have seen some people have with great success with an undergravel filter and sealed glass top on an octopus tank, as well, but most octopuses tend to discover the undergravel plate makes a great cave, and promptly pull the filter apart to crawl beneath it.  This leads to dramatically decreased functionality of the filter, and an octopus that you can no longer see in your aquarium.  Not exactly what most of us have in mind for a pet… 

In any case, regardless of the design you decide to use, I think that using a protein skimmer and power filter with carbon are always a good idea for an octopus.  In addition to the reasons listed above, an efficient skimmer and some carbon will help in the event that your pet is scared into expelling its ink within the confines of the aquarium.  Inking is a normal defensive strategy in cephalopods, and although it is natural it can be a serious concern in a closed system.  The ink itself is not toxic (Wood 1994), but the ink can mechanically coat the gill surfaces of the octopus, and effectively suffocate the animal if it is not immediately removed.  The use of activated carbon and an efficient skimmer can help to remove the ink from an aquarium, and give you more time to discover the problem and do a water change.  In fact, octopuses can control the amount of ink that they expel, and if the amount of ink expelled is small and the tank is large, use of carbon and skimming can completely eliminate the need for immediate water changes (Johnston and Forsythe 1993).  Having said that, however, I always think it is a good idea to err on the side of caution, and I usually recommend an immediate water change in response to inking as a precaution regardless of carbon use.  Also, it should be obvious, but after an octopus has inked, if the activated carbon is doing its job, your carbon will be fouled and needs to be replaced immediately in case your pet inks again.

Flow in an octopus tank

Most species of octopuses like a relatively high flow rate, but that also proves problematic for keeping these animals because most people use powerheads in an aquarium to generate flow, but I cannot really recommend that solution for an octopus tank.  Octopuses have extremely sensitive arms, and they tend to be very tactile animals: like small children, they like to touch, explore, and play with everything they encounter in their environment.  If the impeller of your powerhead is easily accessible (and almost all of them are), powerheads essentially function as a tentacle blender for the curious animal.  The strength of these animals makes properly shielding the intake of a powerhead that much more difficult, because octopuses like to remove them.  In fact, it has been my experience that the various friction-fit filter baskets typically used to diffuse the intake flow (and prevent animals from being sucked into a powerhead intake) are easily removed by an octopus, even after I had glued them in place.  Personally, I prefer to have only an overflow for the open sump in my tank and a remote pump that turns over the tank volume at least 5-6 times per hour.  Even with a mesh-covered overflow, my octopuses have tended to stick their arms into these intakes on a regular basis, so I am very hesitant to use a powerhead in any octopus tank. 

However, having an overflow presents its own set of problems, and need careful attention to prevent disaster.  First of all, the overflow must be covered in such a way that the octopus itself cannot get through the overflow and escape.  As I mentioned previously, even a relatively large animal can squeeze itself through an opening the size of an average adults pinky finger, so you need to make sure that the mesh is small enough to prevent your octopus from getting into the overflow.  Some people accomplish this by using a PVC standpipe and gluing a bioball into the standpipe with epoxy to prevent the octopus from removing it.  Others use a plastic mesh that is held on with a cable-tie or some such thing.  Whatever solution you decide upon, it is important that it 1) is small enough that it does not allow the octopus into the overflow, and 2) is large enough that it does not restrict flow.  The restriction of flow tends to be more of a problem through time than when the mesh is first installed.  As I mentioned above, octopus tend to shed a lot of skin flakes on a regular basis, and if these are allowed to collect on the standpipe drain, there is a good chance that drain will become clogged and your tank may overflow.  This is a serious problem, and the smaller the mesh size used, the more likely clogging will occur, and the more quickly it tends to happen.  Therefore, there is a trade-off between making sure that your octopus won’t escape and making sure that your overflow won’t clog: ideally, you want to use the largest mesh size that you can to ensure your octopus cannot enter the drain for this application.

Feeding your pet octopus

The last problem that I will mention in detail is feeding of your pet.  As I mentioned now several times, I do not consider an octopus a particularly well-suited addition to a reef tank.  In addition to the reasons that I mentioned above, there is the fact that any molluscs, crustaceans and likely even fish in the average reef aquarium will eventually be eaten by an octopus.  This is obviously a concern with any tankmates that you try to add to an octopus tank, but of particular concern in a reef tank, because in general, reef aquarists use a variety of herbivorous snails, hermit crabs or other small crustaceans to control nuisance algae in the aquarium, and these “clean-up” crews are often fairly expensive.  The addition of an octopus to your reef tank will quickly eliminate this population of clean-up animals from your tank, because these are the same animals which are the favorite prey of an octopus in the wild.  Although it would be a healthy diet for your octopus, it is also certainly an expensive way to feed your new pet.  Even more expensive would be the loss of giant clams (Tridacna spp.) that would immediately or eventually prove too tasty a treat for your octopus to resist.  The eventual loss of most of the animals that typically perform as the clean-up crew in your tank, especially when coupled with the additional nutrient input from a relatively large and highly active animal such as an octopus (even a pygmy octopus produces an enormous amount of nitrogenous waste in comparison to a similarly-sized coral), makes it very likely that you’ll experience a pretty significant increase in algal growth in your reef tank. 

Even if you don’t mind having an octopus eat the majority of their tank mates, there are other problems you’ll face with trying to keep an octopus in a reef aquarium.  Even the cnidarians (corals, anemones, hydroids and the like), which an octopus is unlikely to try to eat turn out to be problematic, but in the other direction this time – as I mentioned above, octopuses have extremely sensitive arms, and the constant stinging by the cnidarians is both a source of stress and potential infection for your octopus.  All-in-all, the vast majority of reef aquaria quite simply provide an inappropriate place to try to keep an octopus alive and healthy for extended periods of time in captivity...

So after all that, if you are still determined to get one of these animals, what can I suggest for you to provide the best care possible for your new pet?  First of all, as I’ve belabored in detail above, an octopus needs its own tank, and one that will prevent it from being able to escape.  Second, these animals are active marine predators and they require quite a bit of high quality food.  “High quality food” does not mean feeder goldfish!  Feeder goldfish have approximately 10 times as much saturated fat as any prey item in the natural diet of an octopus, and the continuous input of highly fatty foods into their diet has a dramatic effect on their expected lifespan in captivity (Toonen 2001) .  Although many people are fascinated by the ability of an octopus to hunt down prey, feeder goldfish simply make a lousy food item for any marine predator (Toonen 2001) , and will contribute to an early death for your animal.  So, if you can’t use feeders, what should you feed your octopus?

Well, the natural prey of octopuses consists primarily of other molluscs (especially when young, but in some species throughout their lives) and crustaceans (Boyle 1987).  Obviously, you should make an effort to provide such foods, and you should also try to get the freshest (live if possible) variety of seafood prey to ensure your animals health.  If fresh or live crustaceans are not easily available, the best alternative is to feed your pet good quality frozen shrimp (after you have thawed them, of course), provided that you try to vary the diet from time to time with other prey that is available (e.g., DeRusha et al. 1989).  It also turns out that freshwater crustaceans have a nutritional profile surprisingly close to that of their marine cousins (Toonen 2001).  So, another excellent alternative food for your octopus would be to include a variety of freshwater crustaceans in their diet as well.  Live ghost shrimp are one alternative that usually prove easy to locate (many petshops now carry feeder shrimp) and are easily fed on nutritious fish food prior to feeding them to your octopus.  Another potential for many people is live crayfish, which are seasonally abundant in many areas and often sold at bait shops for fishermen.  Both of these live foods provide the same opportunity for your pet to hunt live food, while also providing a much more nutritious alternative to goldfish.

Enriching your pets life

The reason that I encourage you to find a suitable alternative live prey to feed your octopus is simple.  Aside from the nutritional issue, live prey has another benefit over fresh or frozen seafood: your octopus will have to hunt it.  This may seem unimportant to you, but it turns out to be very important to your pet.  As I mentioned above, these animals are extremely intelligent predators, and spending their life in a completely predictable and unchanging glass box is not only boring for them, it has a strong impact on both their health and behavior (e.g., Wood and Wood 1999).  In fact, during his research with baby octopuses at Dalhousie University, James Wood discovered that he could prevent “suicides” (animals crawling out of their tanks and drying out) by adding sufficient numbers of toys and challenges to keep the animals interested (e.g., Wood and Wood 1999).  Professional zookeepers have long recognized that captive animals housed in unnatural and unstimulating enclosures can develop abnormal, repetitive and neurotic behaviors, and most zoos have made a serious effort to enrich the environment in which their animals are housed to avoid such behaviors (which has led to the development of a trade journal for zookeepers and professional aquarists titled The Shape of Enrichment to exchange ideas to better stimulate captive animals).  Octopuses are no exception to this issue, and a captive animal housed in a tank without sufficient hiding places and without sufficient stimulation can develop a number of stress behaviors including white color patterns, inking, frequent deimatic displays (these are sometimes called “startle flashing” in which false eye spots or brilliant colors are suddenly displayed in an attempt to startle a potential predator), autophagy (eating the tips of their own arms), hiding all the time (depending on species, however, this may be natural in some particularly timid species), and rapid jetting into the side of the tank, among others (e.g., Wood and Wood 1999).  It is therefore critical that you make every effort to try to provide sufficient stimulation to your pet in order to avoid such behaviors and prolong their life in captivity.