A paludarium, part aquarium and part terrarium, with water below and land and air above, is close to perfect moss country. The humid air over the waterline gives moss exactly the damp, sheltered conditions it wants, and the result can be a vertical green landscape running from the water up the back wall.
Where moss thrives in a paludarium
The sweet spot is the emersed zone: the banks, rocks, wood and background that sit just above the water in saturated air. There the moss stays constantly moist without being drowned, and grows lush. Moss fully underwater behaves as an aquatic plant, while moss high and dry near a vent will struggle, so the band around and above the waterline is where it does best. Many keepers run moss from a little below the surface up the hardscape, blending the aquatic and emersed forms of the same plant into one continuous sweep of green.
The zones, from water to air
It helps to picture the tank as three bands. Below the surface is the submerged zone, where mosses grow in their aquatic form, slow and soft. Just above sits the emersed splash zone, kept wet by spray, condensation or the wick of water up porous wood and rock; this is the richest ground for moss, constantly damp in saturated air without ever drowning. Higher still is the drier upper zone near the lid and any vent, where air moves more freely and the humidity dips, and where moss thins out unless you keep it misted. Most of the growth people admire sits in that middle band, so planning the hardscape to put the good surfaces within reach of the water is half the work.
Which mosses
The aquarium mosses double brilliantly here, since species like Java, Christmas and weeping moss grow happily both submerged and emersed, which suits a tank that straddles both. For the purely land portions, the resilient terrestrial cushion and carpet mosses used in vivariums also cope with the high humidity. Java moss is the reliable workhorse to start with, spreading over wood and rock with little fuss and forgiving the odd lapse in care while you learn the tank.
Establishing and maintaining it
Attach moss thinly to the hardscape exactly as in an aquarium, tying or gluing a light layer over wood and rock, since a thick wad rots in the middle. Keep the emersed sections misted or within reach of the splash from a trickle or waterfall feature, which is why paludariums so often include moving water. Bright but indirect light and a humid, lidded environment do the rest. Trim to shape as it fills in, and the clippings will seed new growth wherever they land. For attaching technique and species detail see aquarium mosses.
Light, air and the usual troubles
Moss wants bright but indirect light; strong light bleaches its colour and lets algae take over, while deep gloom leaves it weak and stretched. A little air movement keeps mould and stagnation down without drying the tank out, which is why a small fan or a cracked vent often does more good than harm. The common failures are the same handful each time: moss laid on too thickly rots from within, a patch set too high dries and browns, and rich substrate or trapped debris feeds mould in the still air. Thin the moss, keep the good pieces within the splash, clear anything that starts to fur over, and the tank largely runs itself.