Divide the tank by bedding height, not by a wall
The reliable way to give gerbils both deep digging and a working wheel is to divide the tank by depth. Fill about two-thirds of the length with deep bedding for tunnelling and keep the remaining third shallow, with the wheel living in the shallow zone on a stand or low platform. A solid divider can help during setup, but most keepers find a stepped bank of bedding, firmly pressed, holds the boundary well once the gerbils start work, and the gerbils treat the slope itself as part of the terrain. Trying to run a wheel inside deep bedding fails predictably: one night of digging buries the wheel to its axle, and a buried wheel is chewed, not used.
Build the deep side in layers. Press down a layer of plain paper or aspen bedding, add a scattering of hay, press again, and repeat, because compressed layers with hay woven through them are what let gerbil tunnels keep a roof. Loose, fluffy bedding thrown in to the same depth collapses into the tunnels within a day.
Furnish the dig zone as underground architecture
Bury the furniture in the deep zone rather than sitting it on top. Sink a ceramic or wooden hide so its doorway opens at mid-depth, lay cardboard tubes at angles through the layers, and place a heavy wooden chew partly buried near the slope. As the gerbils excavate, these pieces become chambers and junctions inside the tunnel system, and you will see tunnel windows appear against the glass with rooms opening off them. Leave one corner of the deep zone as the nest site by adding a heap of soft hay and shredded plain paper there; gerbils drag material to a chosen chamber and line it themselves.
Expect constant renovation. Entrances move, the slope erodes into the shallow zone, and part of your weekly routine is pushing bedding back up the bank and re-pressing it, returning the boundary the gerbils have spent the week dismantling.
Keep the wheel zone shallow, hard, and easy to sweep
In the shallow third, keep bedding to a thin layer over the base or use a low platform of wood or ceramic tile as the floor of the zone. Mount the solid wheel on a stand on that platform, sized for a straight-backed run, with enough clearance that kicked bedding falls off the platform edge instead of packing behind the wheel. A sand bath dish belongs in this zone too, on the platform where it stays upright and findable, along with the water bottle fixed above the shallow side so drips and digging never soak the deep bedding or bury the spout.
A food scatter works across both zones: most of the mix over the dig side for foraging, and a small amount near the wheel platform so the gerbils pass through the shallow zone rather than treating it as a corridor. Check the wheel spins freely every evening; a thirty-second check catches the bedding wedge that otherwise silences it for a week.
Cleaning that preserves both zones
Clean by halves. Lift out the wheel platform and sand dish, wash them, sweep the shallow zone fully, and only spot-clean the toilet corner of the deep side, replacing a portion of its bedding and re-pressing the layers with some of the old bedding mixed back in. A full strip of the dig zone destroys weeks of tunnel work and the pair's scent map, and gerbils returned to a completely fresh tank often squabble while they re-establish it. Rotate which third of the deep bedding you refresh so the system is never wholly new.
Draw the zone split once — deep side, platform position, bottle over the shallow end — and keep that layout where everyone cleaning can see it before they start. That shared layout is easy to hold on KinsLe Family Sharing, where the gerbil profile keeps the tank plan and cleaning notes together for the whole household.