What a background actually does for the fish
An uncovered tank is transparent on every side, which for a fish means no wall anywhere: open water in every direction, movement visible behind the tank as well as in front, and light bouncing into reflections that shift as people pass. Many species, especially shy ones and those that live near cover in the wild, read that exposure as vulnerability. A background closes one side of their world. Fish rest closer to it, color up more readily, and startle less at activity behind the aquarium, and aquascapers value the same effect because dark backing throws the plants and livestock into relief.
Choose the kind that suits your tank and temper
The simplest background is a solid sheet, black or deep blue, cut to size; it hides equipment, never dates, and suits almost any aquascape. Printed scenes, rock faces, roots, planted vistas, can look wonderful on a themed tank but fight with the real decor if the styles clash, and they show every bubble and crease. Paint, applied to the outside of the glass, gives the cleanest finish of all and never peels, but it is effectively permanent, so it suits keepers certain of their color. Inside-the-tank three-dimensional backgrounds exist too, though they take up swimming room, complicate cleaning, and are a bigger commitment than most keepers need.
Fitting it flat, on the outside
Fit external backgrounds to the outside of the back glass, never inside, where adhesives and trapped waste become problems. Measure the glass precisely and cut slightly oversize, then trim after fitting. Clean the glass thoroughly first, because every speck shows through. A sheet can be taped at the edges with clear or black tape for a reversible job, or wetted against the glass with a thin film of water and smoothed from the center outward with a card to push out bubbles, which gives a far cleaner look. Work slowly at the corners, since trapped air there catches light and draws the eye. Leave at least the side panels uncovered so you keep your inspection angles and the fish keep a gradient of light and shelter.
What changes after it goes on
Expect behavior to shift subtly over the following days: fish that hovered mid-water may claim spots near the back, shy species venture out more, and colors often deepen as stress drops. Algae will still grow on the inside of the back glass and is slightly harder to judge against a dark backing, so keep up your usual cleaning rhythm and check the rear glass during water changes. If you fit a background while fish are already settled, do it during the day with the room calm, and avoid a full rescape in the same session, since stacking changes makes it impossible to tell which one the fish are reacting to.
A small job worth sharing properly
Fitting a background to a filled, running tank is genuinely a two-person task: one to hold and smooth, one to trim and check alignment from the front. Splitting it, and noting the cleaning change it creates afterward, is the sort of small household job that gets forgotten when it lives in one person's head. Care Teams on KinsLe let you assign the fitting and the ongoing back-glass checks to whoever helps with the tank, with completed tasks marked by a timestamp, so the background goes on straight and stays looked after.