The mourning gecko rewrites normal sizing advice because the population in the tank is not fixed. This small, soft-colored Pacific gecko is parthenogenetic: virtually every individual is female and reproduces without a male, gluing pairs of hard-shelled eggs to leaves, bark, and terrarium corners. Start with two or three in a 12-by-12-by-18-inch tall terrarium and they fit easily; plan instead for the colony they will become, and an 18-by-18-by-24-inch enclosure is the honest long-term size, with bigger planted terrariums absorbing larger colonies gracefully. Expect to rehome youngsters or graduate the colony upward over time; that is normal management for this species, not a failure of planning.
Height is the useful dimension because mourning geckos live on leaves, bamboo, and glass in the upper two-thirds of the enclosure. Floor space mostly serves the keeper, holding the plant pot, drainage, and the little feeding station.
Seal It Like the Babies Are Already There
A mourning gecko adult is about three to four inches long, and a hatchling is a wisp that fits through the seam gaps of many standard terrariums. Choose an enclosure with tight door tolerances, fine mesh too small for a hatchling's head, and no open cable notches, then inspect every corner with babies in mind before the first adult moves in. Silicone or fine mesh can close the small gaps around background edges and door hinges. Finding a baby gecko on the wall behind the terrarium is charming once; as a pattern it means the enclosure failed its most basic job, and a loose tropical gecko in a dry house is unlikely to survive long.
Inside, build vertical cover: slender bamboo, cork tubes, and leafy plants creating layers where geckos rest head-down by day and hunt by night. Cork tubes and curled leaves double as the sheltered egg-laying sites females prefer, and knowing where eggs are glued lets you leave them to hatch in place, which they do reliably at room conditions in the terrarium. Multiple small feeding ledges spread across the upper levels prevent the colony's boldest members from guarding a single food cup, a real dynamic in a group that mixes sizes.
Colony Life at the Larger Size
In the roomier terrarium, set a gentle basking area near the top for daytime warmth choice, ultraviolet light over the basking zone, and shaded foliage below, then mist lightly so droplets bead on leaves for drinking. Mourning geckos chirp and click at each other after dark, posture over favored perches, and raise youngsters communally in the same space, so the larger enclosure is also social housing: space to retreat keeps colony hierarchy from becoming colony stress. Feed prepared gecko diet at the ledges plus tiny live insects, which the whole colony will hunt with surprising coordination across the glass.
Watch the colony's edges rather than its center. A gecko that stops joining the evening feeding, youngsters that never appear at the ledges, tails dropping across several animals, or eggs that consistently fail all point to crowding, a dominant animal monopolizing food, or conditions drifting, and a reptile veterinarian can help interpret colony-level problems if thinning the group and adding feeding stations do not correct them.
Tall, sealed, and planted, the enclosure becomes a small self-renewing world on the wall. Tracking new eggs, hatchlings, and colony size in KinsLe via Family Sharing keeps every family member current on a population that changes itself.