Personally, I'd say anything under 100 liters and the animals aren't worth keeping as pets. I've kept these and like to do one in a 30 gallon aquarium (around 113 liters). If you could go with 150 liters (roughly a 40 gallon breeder), I personally think up to three or four adults would be great in it, and it's much more suitable than even one in a 30 gallon. These guys will live in a lot less water (laboratories keep them in shoebox sized containers, where they can barely even move), they're much more active in larger tanks. In my experience, when kept in smaller spaces, they just sit on the bottom and never move. When kept in larger tanks they move a lot more, and I see them doing more than sitting in the exact same spot all the time, only moving for food. Tank mates are the same way- although they're not necessarily social, if I keep two or more together in a tank, both are out and moving more often. Sometimes together, sometimes at opposite ends of the tank, but kept singly, they seem to sit sedentary more often.
You could cram one in that aquarium, but it would stunt its growth, and it wouldn't ever do more than sit on the bottom of the tank in one spot. At that point, you might as well just get a plastic one, it'll be cheaper, and just as entertaining.
Also, I'd advise against an internal filter. Internal filters tend to make a lot of water disturbance, and these guys don't like that. My favorite is a sponge prefilter (to make regular removal of solid waste easy, making water changes less necessary) on a high-volume canister filter, like an Eheim. Fill it with biological media, and it's great, because it doesn't have to move as much water to keep parameters up the way internal or HOB filters do. Sponge filters work great when the axies are small, but they don't provide enough mechanical filtration to clean up the solid waste larger axolotls produce.
