The short answer for a mud turtle
An adult mud turtle needs at least a 20-gallon long tank, and a 30- to 40-gallon tank with a long, low footprint is the more comfortable permanent home. Most mud turtles top out at roughly three to five inches of shell, which tempts people toward a small display tank, but shell length is not the whole story. This is a bottom-walking turtle that patrols, digs, and forages along the substrate rather than swimming laps in open water. Give it length to wander and it will use every inch; crowd it into a tall, narrow tank and much of that volume is wasted above its head.
Why shallow and long beats deep and tall
In the wild, mud turtles favour slow, shallow water with soft bottoms and plenty of cover. A home tank should copy that shape: water deep enough for the turtle to right itself and submerge fully, but not so deep that reaching the surface becomes work, especially for a smaller or older animal. A long footprint also creates room for a proper warm-to-cool spread, with the basking area at one end and cooler, planted cover at the other. In a short tank those zones collapse into each other and the turtle cannot choose between them.
Laying out the 30- to 40-gallon upgrade
Start the layout with a haul-out the turtle can climb without a struggle — a gently sloped platform or cork piece breaking the surface under the basking light. Add low cover along the bottom: smooth rocks, driftwood, and sturdy plants the turtle can weave through while foraging. Keep filters and heaters guarded and out of the main walking path, and leave one open stretch of substrate so the turtle is not constantly squeezing past furniture. If you keep two mud turtles, step up to the 40-gallon footprint and duplicate the basking spot and hides, because shared pinch points are where nipping and stress begin.
Signs the tank has become too small
Watch for a turtle that paces the glass in repeated loops, digs persistently at corners, or spends nearly all its time on the haul-out rather than moving between water and land. A tankmate that monopolises the platform, or shell and skin staying dirty because waste builds faster than filtration handles it, also points to a footprint problem. Check water quality before assuming behaviour is boredom, though — cramped quarters and fouled water often arrive together. A turtle that stops feeding, develops soft or discoloured shell patches, or has swollen eyes needs a veterinarian promptly, whatever the tank size.
Planning the home around this size
Measure the stand, not just the tank: a filled 40-gallon setup is heavy, needs a level surface away from direct sun, and leaves room to reach in for cleaning without moving lights each time. Juveniles can start in the 20-gallon long, but buying the adult footprint once is usually kinder than two upgrades in quick succession. Once the layout is settled, KinsLe's Family Sharing lets the whole household see the same mud turtle profile and care details, so whoever feeds or checks the water is working from identical information.