What to put in a 125 Gallon Tank?

[SDA]

I would wonder about that too. When I owned my savannahs years ago I had to build a wooden enclosure for each of them and it was 6x3ishx4 feet high because they loved to dig. I would assume ackies are similar in their needs.

[Sauzo]

Fish....

[Regius_049]

I would say the minimum for any monitor (especially if you want to keep two ackies) would be 6ft x4ft x4ft, but at that point you might as well go 8ftx4ftxft. You want at least 2ft of substrate so they can burrow.

Interesting. After starting to read more into ackies (previously it was more just "those are kind of cool" than actually considering them), it seems like they are naturally a solitary species. I had read care sheets that suggested keeping them together was beneficial for the monitors, but after reading some journal articles and better literature, this appears not to be the case. Thus, now I would only be considering a single ackie monitor. I have never really liked housing animals together, so I am actually kind of pleased with this development.

It is interesting to find myself on the "smaller cage" side of the argument as it seldom seems to happen, but after consulting the German guidelines (I like to refer to these). For monitor species (terrestrial), they recommend a 5 x 2 x 2 KRL. More specifically, assuming an ackie SVL (snout to vent length) averages out around 24mm (~9.5 inches) with a total length close to 63cm (~24 inches). Reported lengths of ackie monitors suggest a range of 24-28 inches generally, so this assessment seems reasonable. As an outlier, I can also assume a very large male specimen, I think 34 inches is the largest anecdotal report I have seen and using the same ratio (9.5/24) I would get about 13.5 inches SVL.

Given those numbers, the enclosure size I arrive at would be: ~68" x 27" x 27" for the abnormally large specimen or 48" x 19" x 19" for the more average (24 inches) report. Thus, somewhere between those numbers should be reasonable using this metric. If I drop myself somewhere in the middle, I could propose a 5' x 2' x 2' enclosure. According to H. Cogger's book, ackie monitors are less active than other monitor species and have a lower metabolic rate (though I suspect higher than many other lizard species). They appear to be sit-and-wait predators, hiding in or under rocky crevasses and formations.

Now, I will say that after perusing a variety of youtube videos of ackie monitors, they do seem rather active and larger enclosures look "more appropriate" when considering an adult ackie. When considering a thermal gradient, I am pretty certain I could establish whatever gradient I needed in a 5' enclosure. Generally, I read ackies like a hotspot of 110-170 (this number seems to vary quite a bit from report to report) and an ambient of ~80-85 or so. A retes stack appears pretty popular and I don't see an issue creating such a setup. Climbing surfaces seem a given for enrichment, but I am unconvinced that ackies are true burrowers. Clearly they do dig, but it appears to be more of a foraging or nest building behavior. They likely use it in the wild as a source of moisture or humidity as water can be scarce in their natural habitat, but it seems what they really want is a place to hide / wedge themselves into. I can find no report that ackies "live" in burrows like a frog-eye gecko or gila monster. They more utilize them for a specific purpose. I do not note this to discourage a substrate that allows digging, simply that 2' might be unnecessary. That said, given the size of the lizard and the fact that they do dig, it does seem at least a 12" layer of substrate is appropriate. This is deep enough that they can dig and burrow to a degree if they so wish.

As a reference, assuming you have an argentine B&W tegu (50 cm SVL or ~20"), using the same guide as above would give you a roughly 8.33' x 3.5' x 3.5' enclosure, which is pretty close to your setup even though you feel it may be too small at times. 8' x 4' does appear to be the "standard" for the B&W tegu as far as receomendations go.

In short (TLDR), given the guideline and some species appropriate adjustments: I would arrive at a cage of 5' x 2' x 3' (pretty close to my original estimate). This would give 5' x 2' of floor space as well as 2' of height and 1' of substrate. Feel free to critique my assessment. You are of course, more than entitled to your opinion and I am happy to see people advocating for larger enclosures, but I do think I can assert that an ackie monitor would still thrive in the enclosure I have proposed.