It is typically recommended that the tank size be at least fifteen to twenty gallons for one adult axolotl. Tank size should be increased ten gallons with each additional axolotl. Generally speaking, bigger is better. However, keep in mind this is not a hard fast rule but just a recommendation. Different factors can come into play including the size of your axolotl(s), how much filtration you have, and how often you are performing water changes and spot cleaning. Axolotls have a high bio load. Thus, ammonia spikes are to be expected every now and then. In my personal experience, larger tanks are easier to control water parameters in versus small volumes.
When choosing your tank location make sure you are putting it on a stable surface. It is best to avoid it being near bright windows as axolotls prefer the dark. (Note: Some specimens are more sensitive to the light than others.) However, this can be avoided by constructing a make shift curtain for during the day if no other options are available. Many people prefer to keep their axolotl in the basement away from the sun. This is ideal if you have a finished basement you spend a lot of time in. However, this is not an option for us so we keep them in our media room where we spend the most time.
In terms of tank material, glass and acrylic are the most popular. We prefer acrylic because it is significantly stronger and weighs less than glass. It is also known for having excellent clarity. One downside to acrylic tanks is that it is more prone to scratching. We recommend using a microfiber cloth for cleaning.
As for tank shape, the more floor space the better. Think long, low, and rectangular. This is because axolotls typically spend large amounts of time on the tank floor. Round, hexagon, bow fronts, etc. are not recommended. If picking a taller tank, just make sure you have lots of tall decor/plants they can chill on to maximize their ability to explore and hide.
Lastly, it's recommended that the tank either has a secure cover or that the water level is maintained three inches below the top of the tank. This is because axolotls can jump out. We prefer a secure cover over lowering for peace of mind :).