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Introduction to the Nitrogen Cycle (Or, Why You Killed That Goldfish When You Were Little)

Most first-time fish owners kill their pet within a month of bringing it home. This is because most pet stores fail to educate their employees and customers about the nitrogen cycle, a crucial component of the aquatic ecosystem. This beautiful example of commensalism (a symbiotic relationship that benefits all of the organisms involved) is what makes life sustainable for aquatic animals.

The first thing to understand is that an aquarium is not just a box with water in it: it is a miniature replica of the ecosystem that allows life to flourish in our rivers and oceans. The water is full of microorganisms, and some of them have extremely important roles to play.

Meet the nitrifying bacteria:

In the aquarium, just as in nature, animal waste and decaying organic matter constantly release ammonia (NH3) into the water. Ammonia is highly toxic — even a trace can be harmful to most aquatic species. Luckily, bacteria like nitrosomonas are able to use the oxygen present in water to convert ammonia into a less toxic compound: nitrite (NO2).

Note that “less toxic” does not mean “harmless”. Animals tolerate nitrites a bit better than ammonia, but as it builds up in the aquarium (or any other body of water), it can still lead to burns, illness and death. Thankfully, that’s when nitrobacters come in to save the day. Using oxygen, these little guys convert nitrites into nitrates (NO3), which are far less toxic than the other two compounds.

In nature, nitrates don’t really have a chance to build up because plants and algae use them for growth. If you keep plants in your aquarium, they will “eat” some of the nitrates for you. The leftovers will become algae food, which is why you may notice an algae bloom if you haven’t changed the water in a while.

If nitrates keep climbing, the plants and algae may not be able to keep up, and your animals are likely to get sick. This is where you come in! Regardless of the species you are keeping, you should be doing partial water changes at least once a week. It’s a good idea to test for ammonia, nitrites and nitrates once in a while to make sure that your microorganisms are doing their job and keeping your aquarium water safe.

Which brings me to the main point of this post: brand new, clean aquariums are unsafe.

More specifically, the problem lies with the filter. When you first obtain it from the store, the filter is bacteria-free. This means you need to install them yourself and give them time to make themselves at home before you can safely add any animals. We call this process cycling. It takes a while and it’s kind of tricky, so people who don’t understand how important it is tend to skip this step. Then, as ammonia builds up, their animal’s health begins to deteriorate. This is such a common phenomenon that it has been given the name new tank syndrome.

Now that you know why cycling is so important, here’s how to do it properly.

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Can I house two axolotls together? Will they fight?

Axolotls are not a social species, so they don’t need a friend to be entertained. That being said, it is perfectly ok to house axolotls together, as long as they are roughly the same size. If one axolotl easily fits into the other’s mouth, chances are it will end up as a snack. Usually, axolotls grow out of their cannibalistic phase once all four limbs are formed, but some unfortunately retain those instincts. If space is very limited, you might see some snapping behavior at feeding time, which can lead to injuries. In addition, if your axolotls are not adequately fed, they may eat each other’s limbs for extra nutrition. Those limbs will grow back, but do your best to limit altercations by feeding them a nutritious diet and keeping them in proper housing conditions.

Keep in mind that if you house a male and a female together, they will most likely end up breeding. The young larvae are not that easy to care for, so make sure you know what you’re getting into before you choose to attempt it!

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Are axolotls difficult to care for?

Axolotls are very easy to care for if you have the right setup. Given proper housing conditions, all you need to do to keep your axolotl happy is to feed it and keep its water clean.

An adult axolotl only needs to eat one live earthworm every other day (the kind used as fishing bait all over the country). The axolotls I sell are also trained to accept pellets. Those are a convenient backup, but I still recommend feeding earthworms if you can, as they don’t get the water dirty the way pellets tend to.

Spot cleaning
Whatever you choose to feed your axolotl, within a day or so it will all come back out as one big solid poop, which is easily picked up using a turkey baster. I do recommend picking up the poop as soon as you spot it, since axolotls are silly creatures who will put anything in their mouth. If they eat their own poop by mistake, they will do a spit take and scatter bits of poop everywhere!

Weekly maintenance
Whether your axolotl makes poop rain or not, you will need to do weekly water changes. Use a siphon to remove 1/3 to 1/2 of the water, and refill with dechlorinated water (or water from the tap, with some conditioner added).

That’s it! As far as pets go, they are not very demanding at all, but you do need to be consistent about these three things.


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Axolotl housing

Proper housing is the key to keeping your axolotl healthy! Please make sure to read all of this before buying an axolotl ᗒ(◍•ᴗ•◍)ᗕ

Aquarium size
The general rule of thumb is to allow a minimum of 10 gallons of water per axolotl. That being said, the important thing is to offer sufficient floor space for your axolotls to be able to move around comfortably. Axolotls typically reach an adult size of 10 to 12 inches, and spend most of their time walking along the bottom of the tank, rather than swimming. Here are my stocking recommendations for some common aquarium models:

  • 10 gal (~20″ x 10″ x 12″): 2 juveniles, or 1 dwarf/mini axolotl
  • 15 gal (~24″ x 12″ x 12″): 1 to 2 adults
  • 20 gal high (~24 x 12 x 16): 1 to 2 adults
  • 20 gal long (~30″ x 12″ x 12″): 2 to 3 adults
  • 30 gal (~36″ x 12″ x 16″): 2 to 3 adults
  • 40 breeder (~36″ x 18″ x 16″): 4 to 5 adults
  • 55 gal (~48″ x 12″ x 21″): 3 to 4 adults
  • 75 gal (~48″ x 18″ x  21″): 5 to 6 adults
  • 120 gal (~48″ x 24″ x 25″): 7 to 8 adults
  • 180 gal (~72″ x 24″ x 25″): 11 to 12 adults

Axolotls prefer very slow-moving water and will become stressed to the point of illness if the water flow is too strong. They are very light and easily pushed around by the current. Signs that your axolotl is stressed by the water flow include forward-curled gills, sliding/slipping, and staying in one spot (particularly in a cave or tank corner.)

To avoid this issue, choose a gentle filter such as the ones advertised in my shop section — they are all tried and true axolotl-safe models! Avoid HOB (hang-on-back/power) filters and canisters without adjustable flow, unless you are prepared to turn them into a DIY project!  Already bought them? See my tips & tricks section for ways to turn them into a super efficient, axolotl-safe system. [Coming soon!]

No matter which filter you choose, you will need to provide a source of biological filtration. Just like fish, axolotls are prone to new tank syndrome, which means proper cycling is absolutely essential. If you’re not sure what any of  this means, please read this: Introduction to the Nitrogen Cycle (Or, Why You Killed That Goldfish When You Were Little).

Substrate (or not?)
Contrary to popular belief, axolotls do just fine in a bare-bottom tank, as long as the water flow is sufficiently gentle. I personally prefer to keep my tanks bare, as I find it easier to clean. If you prefer to use a substrate for esthetic reasons, keep in mind that both sand and gravel are now known to pose an impaction risk [article coming soon!]. If you choose to use sand, make sure the grain is small and smooth, and that the material is non-clumping. Types of sand successfully employed by aquarists include:

  • Caribsea Super Naturals Moonlight Sand (stir weekly or use a thin layer)
  • Caribsea Sunset Gold (stir weekly or use a thin layer)
  • Seachem Meridian (not as smooth, but very small and carbonate-based — recommended as buffer if your water is on the soft/acidic side — stir weekly or use a thin layer)
  • Play sand (available at hardware stores — larger grain, requires heavy rinsing — use with fully grown adults only)
  • Pool filter sand (available at pool supply stores — larger grain, requires heavy rinsing — use with fully grown adults only)

Gravel is unsafe at nearly every stage of the axolotl’s life. Remember, axolotls are like babies: if it fits in their mouths, it will end up in their mouths! If you really want to use rocks, use large, smooth pebbles or river rocks that are significantly larger than your axolotl’s head, to make sure they won’t be ending up as a snack.

Axolotls like to explore their environment, so it’s a good idea to switch up their decor once in a while to keep them entertained. They like to have objects to climb, floating items to cling to, and places to hide. Pvc pipes, clay pots, caves and driftwood are all appreciated. Make sure all of the decor pieces are smooth, and too large to get accidentally swallowed. Plants are fine (plastic or live). Axolotls are carnivorous, so they have no interest in eating live plants — they just enjoy hiding in them or clinging to them for support. I recommend getting the floating kind, as they also provide shade, and will stay out of your way during water changes.

Lighting (or not?)
Axolotls don’t particularly need lighting, and will shy away from bright lights. They don’t have eyelids, which makes them easily blinded by sudden light changes. Their vision in general is poor, and they hunt largely by following smells and reacting to movements in the water. If you need to use lighting for plant growth or picture-taking, just make sure you provide your axolotl with some shaded spots to hide in.

Axolotls are subtropical animals, which means they should never be kept in a heated aquarium. They can handle temperatures between 4 and 22°C, with 15 to 18°C being their preferred range. At 23°C, they start to get uncomfortable and often refuse to eat. At 24°C and above, the stress can cause them to get physically ill, and eventually cause death. If possible, try to keep your aquarium in a cool part of the house. During summer, you may need to cool the room down with air conditioning. A regular desk fan or a small clip-on fan may help lower the aquarium’s temperature by a few degrees. If this isn’t sufficient, you may need to purchase an aquarium fan or chiller.

Cover (or not?)
Axolotls rarely jump out of their aquariums, but it can happen. If you choose not to use a cover, make sure the water level is low enough to discourage jumping.

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How do I know if axolotls are the right pet for me and my family?

Do you want a pet that is very active?
Axolotls are sit-and-wait hunters. They don’t typically spend a lot of time swimming around. Mostly, they’ll walk around the tank, climb the decor, or use it as a flotation device. Their goofy poses can be entertaining, but if you want an aquatic pet that moves around a lot, it’s best to stick to fish. On the bright side, their tendency to stay still or move slowly makes them ideal photography subjects!

Are you able to keep their water cold?
Axolotls do best at temperatures between 15 and 18°C.  They can tolerate a range of 4 to 23°C, but at 24°C and above, they could get sick or even die. If your house gets very warm in the summer, it may be necessary to cool the water with air conditioning, fans or an aquarium chiller.

Do you understand the nitrogen cycle?
Before getting any aquatic pet, it’s important to understand the basics of keeping them alive in an aquarium. Please take a moment to read about the nitrogen cycle, its relation to new tank syndrome, and the proper way to cycle your filter before bringing an axolotl home.

Can you make a 10 year commitment?
Given proper housing conditions and care, axolotls can be expected to live 10 to 12 years in captivity. If you are buying an axolotl as a pet for your child, please make sure you are prepared to care for it as well!