At Least 20 Gallons For One Axolotl.
You Need Biological and Mechanical Filtration.
No Gravel, Gravel Can Lead To Impaction Causing Death
The Aquariums Temperature Should Never Exceed 70 Degrees Farenheit.
Lots of Hideouts.
Don't Place The Aquarium Where Temperature Fluctuates.
No Bright Lights
To keep a singular axolotl you will need a 20 gallon tank.
If you are going to keep more then one axolotl in the same aquarium multiply the number of axolotls by 10 and add 10 to the product. The number is the amount of gallons needed. (10a + 10 = g)
For example, if you want to keep three axolotls you will need 40 gallons. 10(3) + 10 = 40
Any filtration that supports the nitrogen cycle is great, just remember axolotls don't like that much water flow.
My personal favorite filters are sponge filters and canister filters.
This is a great video to learn about the nitrogen cycle.
Watch this video to find the perfect filter.
Since axolotls need water that is 60-66 degrees fahrenheit their water will need to be chilled.
There are many ways to chill aquarium water including:
Investing in a water chiller to precisely cool your water. This method is expensive and also needs a pump or canister filter to get the water from the tank to the chiller and back to the tank.
Putting frozen water bottles in your aquarium. This technique may not precisely cool the water but it is a inexpensive way to chill the water. Although you will have to remember to change out the water bottles and refreeze them frequently.
There are only a few good substrates for axolotls.
Sand is a great option for axolotls over 5 inches because it gives them a great surface to stand on and is completely safe as acidental consumption will not cause blockage in the digestive track.
Another good option is to have no substrate. This is good for axolotls under 5 inches and it makes the tank very easy to clean.
Just to note one substrate you should never use is gravel. This is because if the axolotl consumes a pebble it may get stuck in the digestive track and cause impaction, this may lead to your axolotl dying.
There is a lot you can do for decor like live/fake plants, driftwood, hideouts, etc.
As long as there are a handful of hidouts your axolotl will be happy.
You can use a light for plants, viewing, etc as long as it is not extremly bright.
Lighting is not necessary in a axolotl tank because it may stress the axolotl.
Your axolotl tank should be placed where there is little temperature fluctuation. Do not place your tank next to windows or doors leading to the outside.
The information listed above concludes my recommendations for a great axolotl tank. However, I strongy advise that you do your own research!