I will warn you...
This is a controversial topic, but I'm here to give you my take for what it's worth :)
It's on the topic of gravel for axolotls.
Let's get into it!
Creating a Natural tank
When considering the best substrate for an animal, it is always a good idea to consider their original natural habitat.
While there are no axolotls in the wild, that's where the ones we have originally came from.
Animals thrive when their environment is most like the one they would have in nature.
This begs the question...
What would the axolotl live on if it lived in the wild?
Is gravel a safe option for the axolotl tank?
The Axolotl's Wild Substrate
Those of you who have ever been to a natural body of water in North America may remember finding some mud at the bottom.
But if you study these habitats closely, you'll see there are generally a variety of stones to be found in the sediment.
It's actually not normal to have a pure mud bottom.
And of course, there are natural fauna and flora to be found - including small snails (which so many say is a choking hazard).
The bottom line?
Axolotls would have access to some rocks if they still lived in Lake Xochimilco's complex of lakes and canals in Mexico, where they were natively from.
A recreation of a Xochimilco biotope here for a 2017 competition includes soil... AND "small cobble" (aka small stones) as per their substrate description.
This next one to me is incredibly interesting.
A 2019 biotope for Ambystoma dumerilii (which is a species of axolotl very similar to Ambystoma mexicanum also known as the "highly endangered Lake Patzcuaro axolotl," and is also found in a high altitude lake of Mexico, just a 5 hour drive away from Lake Xochimilco in Mexico City) includes an abundance of small stones, seeds and little snails visible in the following video:
The aquarist gives a description of Pátzcuaro lake's substrate, home to dumerilii:
"The bottom in general is a mixture of clay and very fine sand, rocks of different shapes, sizes and porosities, you can also see parts of pine seeds and decaying leaves, as well as branches that have fallen to the bottom, the lake’s riverside does not have steep slopes..."
(For what it's worth, this entry placed #10 for the 2019 competition out of 135 entries.)
Finally, here's a 2015 entry that took #3, a recreation of a portion of the axolotl's original habitat:
These contestants studied intensively to recreate the environments of their entries as closely as possible to the native ones, some even gathering materials from the areas of the lakes that were not as heavily polluted.
Now before you say "Well, we don't know how long the animals remained happy and healthy," the point is not how long they were happy, but how the aquarium owners did their very best to recreate the actual substrate of the amphibians in the wild, and in doing so they all decided to include various sized small hard objects.
Which brings us to the next point...
The Benefits of Gravel Gastroliths for Axies
1. May Help Prevent Floating Problems
Many people who keep axolotls find that they often have issues with floatiness and excess buoyancy.
It's thought that air bubbles in the gut can cause this.
But usually these people with floaty axolotls don't let their animal live on a substrate that includes gravel.
Could it be that these help weigh them down and, in some cases, help prevent floating issues?
It likely has something to do with something called "keel."
2. May Aid in Swimming Abilities
Scientist Natalie K. Gordon, PhD has extensively studied axolotls and reports her findings in studying their relationship with gravel:
"As a scientist, one should never just accept received wisdom at face value, and so, since I was the axolotl expert in the collaboration, I got to do a full literature review on the topic. The result was that I learned there is absolutely no proof whatsoever that axolotls get gut impaction from eating gravel. I also learned all about gastroliths. A gastrolith is a rock ingested by an animal on purpose and carried around in the gut to provide stability, a weighted keel, while swimming. A clever pair of paleontologists explained that the little piles of rocks consistently found in the belly area of fossils of plesiosaurs were gastroliths. They pointed out an animal without a keel could compensate for the lack of a keel while swimming by having weight in the gut. Without such a weight, you end up rolling in the water as soon as you pick up any speed. Fish have a swim bladder that makes the top lighter than the bottom and they have fins. Plesiosaurs swallowed rocks. After that, I found examples all over the scientific literature of gastroliths including a complaint from a fellow who was dissecting frog tadpoles for an unrelated reason about how you had to be careful because the tadpoles had little rocks in their guts that can wreck the very expensive fine blade of a scientific microtome. Even seals apparently swallow gastroliths.
I did one final test. Baby axolotls swim, continuously hunting prey. We kept our baby axolotls in 25 gallon “pond tanks” chock full of things delicious (if you are an axolotl) to eat like Daphnia. My hypothesis was that the wee little 2-3 cm axolotl baby that has just grown front and back feet should really need gastroliths and therefore have a strong instinctive drive to find them. I put a tiny 3cm dish of fine coloured gravel in one bare corner of the 25 gallon tank, far from any food. Sure enough, I kept finding the little guys sitting on the gravel. When I offered them an assortment of sizes, each size a different colour, they would take the tiny rocks in their little mouths, wriggle their mouths in the cutest axolol style smile and swallow some and spit out others. I could see the coloured stones in their transparent guts and they almost always took stones of one colour. They even have a size preference. I also found mucus and waste with stones all over the bottom of the tank. So it appeared the baby axolotls ingested the gravel deliberately and passed it at random. So gastroliths don’t stay in the gut. They pass through the gut and the animal has to constantly seek out fresh gravel. Gizzards of birds probably evolved for more efficient retention of gastroliths. I also compared the swimming behaviour of baby axolotls who had access to gastroliths with those who did not. The ones with bright coloured gravel swam straight and fast. The others often missed prey because they rolled out of control on the big rush." (Source: https://embryogenesisexplained.org/2015/09/10/gastroliths-how-i-learned-to-stop-worrying-and-love-gravel/)
I highly recommend reading her article, it's well worth it :)
What about Gravel Stone impactions?
Howls of "death by impaction and infection" and accusations of poor animal husbandry frighten most new axolotl owners into having only a bare bottom, tile or fine sand substrate.
Actually, the scientist referenced above has a good explanation in the article linked above.
What it boils down to is when people don't know for sure what's wrong, a lot of times they assume it's the gravel because they believe it's abnormal and/or dangerous for an axolotl to eat such things.
So gravel gets the blame, when in reality there may be other things at play that went undetected.
On the flip side...
Because people are trying so hard to make their tank as free as possible from small stones, their axolotls often get desperate to satisfy their instinctive desires for gastroliths.
What this means is they can end up eating stuff they normally wouldn't, which can cause intestinal problems and even death.
- They may eat snails, if there are any to be found.
- They may eat plant material.
- They may eat foreign objects.
- They may eat large pebbles that are unsuitable for them.
You know how people who were starving in the past would eat shoes, candles and other weird things that were dangerous?
The same thing may happen with axies when they don't get their gastroliths.
So it can be more dangerous to have them in with no substrate, if they end up swallowing something else weird.
The bottom line?
By trying to prevent impactions by depriving them of gastroliths, the animal may actually be placed at a greater risk of getting them.
I have heard the story several times:
"I got an axolotl that wasn't eating/throwing up and pooped out a bunch of gravel/sand and it took a while but now they are back to normal, therefore the sand/gravel was making them sick."
For this I have two words, which we must separate:
Correlation vs. causation.
Just because something correlates does not mean it was the cause.
The thing is, there are lots of things that can cause an axolotl to lose its appetite.
Stress is a big one and can be caused by a variety of factors
The presence of gravel does not mean it is the primary cause of the issue - it can to be there when the animal is sick, but gets the blame because the owner does not know the reason why animal was sick to begin with, or overlooked it because of the gravel gut.
Axolotls Love to Eat Rocks!
Let's face it:
Axolotls have an incredibly strong drive to swallow small hard roundish objects.
When given access to various sizes of gravel, they will spit out the ones they don't want and swallow the ones they consider to be the perfect size.
Their bellies may become darker from the rocks inside.
This habit seems weird to us as people.
When we think about swallowing rocks or course sand, we probably feel pretty repulsed, and for good reason - such things are bad for people.
But what is bad for people may be good for some animals!
Axolotls have such a desire to eat stones that without them, they may even start swallowing other things that they wouldn't normally eat to try to get that effect.
Axolotl owners will attest to the fact that axolotls will swallow all kinds of foreign objects.
Could they be doing this because they are being deprived?
This can lead to problems as we'll discuss later.
Why do Axolotls Want to Eat Rocks?
This may seem strange at first, given their meals in the wild would generally consist of worms and other live aquatic organisms.
So why do they do this?
Most people chalk it up to the animal just being dumb.
Or trying to hunt for food and being unable to distinguish rocks from food.
So they decide to make their tank as bare and smooth as possible, fearing every object that could possibly fit in their mouths bigger than a grain of fine sand, some even going to great lengths to glue down their entire pebble substrate (or stripping it all out).
They may reason that the animal in the wild would live strictly on a mud surface and wouldn't have access to any such objects naturally.
But is this right?
We learned that the habitat of the axolotl most likely includes small stones among the sand and mud, not just mud alone.
And if axolotls were all "dumb" to eat them, they would have not survived very long.
I believe that like marine reptiles and other amphibians, the axolotl purposely eats rocks to utilize them as gastroliths.
What's a gastrolith anyway, and why would an axolotl want them?
What Size Gravel is Ideal for Axolotls?
It depends largely on the size of the axolotl.
As the cited scientist above notes, axolotls have their own size preference.
I can't give you an exact size because it likely varies from lotl to lotl...
... But here's my opinion:
If only one kind of gravel will be used, I would choose a smaller fine gravel that is approximately 2-4 mm in size, rather than 1/4" pea gravel only, which may be too large.
Fine gravel is easy for the animal to pass (especially smaller ones), but helps provide keel.
That said, having more than one size is good.
Then they can pick out what they like.
I think you can still use pea gravel, but I believe if you are going to have this or larger size gravel than this, it is important to also provide smaller gravel so the axolotl has a choice.
Basically, providing variety.
There is a substrate in Germany called Axogravel.
It is essentially smooth small stones that range in size from around 1-3mm.
In the US, a great alternative - which I personally use - I have found to be Pisces silver pearl or gold pearl aquarium gravel.
The teeny pebbles herald from New Zealand and are beautiful, very smooth and can be used as gastroliths.
It's also an excellent choice for a planted tank as it supports delicate root growth.
Let the animal decide what's best for them.
In nature, that's how it would work.
If you only provide gravel that is too large, or only big pebbles, the axolotl may not find the gravel that is the right size and force itself to eat larger gastroliths than what it should, (or other weird things) which could lead to impaction.
As a side note:
Some people find it strange that their axolotls dig around in the substrate.
And it would be a bit odd for an animal that does not forage to find its food, but waits for the food to float by and snap at it.
The digging pattern seems especially common in tanks with sand only.
Owners may bewail how the axolotl is messing up their tank, but could it be they are trying to get past the layer of sand in the hopes of finding gastroliths?
Something to think about.
Axolotls are a misunderstood animal in many ways, but I believe by observing the natural environment of these animals as well as scientific research, we can care for them properly.
I understand not everyone will agree with me about this.
That's fine! To each their own :)
Thanks for reading.