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Axolotls, Ambystoma mexicanum, also known as the water dog, Mexican salamander, and Mexican water dog, is one of the most unique aquarium inhabitants out there. They are a salamander that stays larval its whole life, meaning they remain fully aquatic and retain their external gills through adulthood. They can grow quite large, up to 12" with reports of the rare specimen getting even larger. They are native to only a couple lakes in Mexico, and their natural range is under extreme threat. Fortunately for them they are a very popular species used in laboratories all over the world because of their regeneration abilities (they can regrow their gills, tail, and even limbs). This means they have a very large captive population and are understood extremely well. They also reproduce very easily in captivity. This has resulted in one of the most interesting aspects of keeping them, they come in a wide range of colors. These include wild type (blotched olive and gray), melanistic gray (solid gray), melanistic black (solid black or dark gray), white albino (white with red eyes), leucistic (white with black or blue eyes, frequently with black peppering on the body), gold albino (yellow body, red eyes, gold blotches on body), and the almighty piebald (white with large blotches of black, like a cow) which doesn't seem to be available anymore. There is now also a genetically modified variety that has a green fluorescent protein (abbreviated GFP, such as albino GFP). These literally glow green under a blacklight.
Although extremely interesting, there are a lot of things that limit their compatibility in aquariums. Most importantly is temperature, they cannot be kept over around 74F. Anything higher causes severe stress and a failure to thrive. Unfortunately this is about the coolest that most homes are kept in, making room temperature their maximum, and too high in some homes. Their size makes them a threat to most aquarium fish. Adult axolotls are very large and can easily swallow most aquarium fish whole, which they are happy to do at night when the fish are sleeping on the bottom and the axolotls come out to wonder around the bottom. If the fish are too large to be eaten (or the axolotls are still young) the axolotls make easy targets and their fins are nipped at with ease. The fish can dart in to take bites out of the gills and get away before the axolotl can react. Although the gills do regrow, it is simply cruel to keep them under these conditions. So if you combine the temperature, size, and behavioral compatibility issues you are left with effectively no possible tankmates, which is exactly what most people will tell you to keep them with, nothing. Even other axolotls are not an option unless they are all roughly the same size since large axolotls will eat smaller individuals, or at least tear off body parts like gills and limbs.
However, I have kept axolotls with fancy goldfish. In my experience the goldfish are too large to be eaten by the axolotls and too slow to nip at their gills without being nipped in return. This results in a comfortable combination if the tank size allows. The only issue is that both species get quite large and therefore the tank size will limit how many of each you may keep.
There are some other issues more important to keeping axolotls than fish. Aeration is vital, especially if your tank is anywhere near their upper limit (anything in the 70s). A good strong air pump will help increase aeration as well as provide gentle flow that is not stressful to them. Strong flow can be stressful so if a high flow filter is used (like a canister or large hang-on-back) steps should be taken to reduce the overall flow in the tank. This could be using a spray bar on the canister, pointing the output directly at the front, sides, or along the back of the tank, or having lots of decorations to break up the flow.
Substrate is another issue that is particularly important to axolotls. Since they actually walk around on the bottom it is more than just the aquarist's preference. Bare bottom can cause sores to develop on the tips of their toes and be stressful to the axolotls since they have no grip. Gravel is also a no since they can swallow it and become impacted, which can be lethal. Sand is the only option since it gives them something to comfortably grip. Fortunately it is also the best option for almost any aquarium since it is so clean, natural, and safer. For more information read the article on Sand as a (Superior) Substrate.
Lighting can be an issue as well. Intense, bright lighting can be stressful since axolotls are nocturnal. I kept PVC pipes and connectors for my axolotls to hide in during the day. When they were smaller I also used ceramic aquarium decorations made to look like caves and hollow logs. Both worked very well, the axolotls loved them and they were smooth enough to not cause any scratches or abrasions. I had enough light on my tank to grow freshwater plants and my axolotls didn't seem any more bothered by it than they woudl have been with any light.
Feeding is always an important issue with any animal. Axolotls are usually fed salmon or trout pellets in labs. I have found New Life Spectrum to be at least as good since mine were the most robust ones I have seen in person, and grow as well as the breeder's did. Many people like to offer meaty foods for some reason. These include frozen foods, earthworms, and even strips of meat. This is NOT necessary at all and is nothing more than a protein source. They should grow very well on a high quality pellet, adding meaty foods will simply throw off the balance of nutrition in the pellets.
Water quality is just as important to axolotls as it is to fish. Like many fish the exact pH and hardness are not important, water quality must be kept as high as possible though. Please read the article on Water Changes and Water Quality in the Aquarium for more information.
Tank size is not a major issue, but certain minimums should be kept. I would consider a 20long to be the bare minimum for one, a 30long minimum for a pair. Larger tanks will allow for more axolotls and goldfish. There should be more hides than axolotls to make sure they can avoid eachother if needed.
Axolotls can be sexed by looking at the genital area. Males have a noticeably swollen cloaca while females have a smoother appearance to the area around the cloaca. Stimulating mating is done in ways similar to many fish. By simulating seasonal changes they would experience in the wild (photoperiod, changes in temperature, etc.) they will be triggered to breed. By shortening photoperiod followed by increasing it again or slightly raising the temperature followed by dropping it (simulates onset of rainy season) you can mimic these natural events. The male will deposit spermatophores (packets of sperm) on the bottom of the tank and then lead the female above them at which point she will pick them up with her cloaca. Later she will lay the eggs individually, preferably on plants (fake are fine, they are easy to remove from the tank and will not rot). The eggs can be reared in a separate tank. After a couple weeks they will hatch and the offspring need to be fed live foods as would be done with many fish with very small fry. Later they will accept non-living food items.
Getting a hold of axolotls can be challenging at best. Most stores never carry them, and most of them won't know what you are talking about. The best sources will be reptile expos (and even these are quite limited, one may always have one vendor with some, another may never have them) and online retailers. Buying online direct from a breeder is the best way to get a good price and a selection of colorations. Websites like FaunaClassifieds.com and Caudata.org would be the best places to try to find a breeder. I got mine from Michael Shrom, based in Pennsylvania. He has had a range in colors, including green fluorescent protein, great prices, and is willing to ship. He has been available on the mentioned sites.
Overall axolotls make a very interesting and eye catching inhabitant to an aquarium, but they do have specific needs that must be met for them to thrive.
If you are interested in purchasing any axolotls I got mine from Michael Shrom and was more than happy with the quality of the animals.
You can contact Michael Shrom at: email@example.com Email Him Now