Zebra danios live in the upper half of the tank, racing back and forth in a loose school, so their substrate has a supporting role rather than a starring one. Smooth, rounded gravel or sand is the sensible choice: easy to clean under a high-energy group, safe for any bottom-dwelling tankmates sharing the floor, and steady enough to root background plants.
What Danios Actually Need From the Floor
A danio rarely touches the substrate outside feeding time, when stray food is picked off the surface. That means grain size matters less for the danios themselves than for water quality and for tankmates. Rounded gravel will not cut a resting fish below them, and neither gravel nor sand should be so coarse that food vanishes into deep gaps, because a school of danios produces a steady rain of missed crumbs that will foul hidden pockets.
Colour is worth a thought. A mid-tone or darker floor sets off the horizontal striping of a zebra danio and gives the school a calmer backdrop under bright light. Very pale floors reflect light upward and can make fast fish look washed out, though the danios themselves will stay active either way.
A Special Case: Breeding Over Gravel
Zebra danios scatter eggs as they spawn, and in a community tank most eggs that land on an open floor are eaten. In a dedicated breeding setup, a layer of large, round marbles or very coarse rounded stones is sometimes used as the floor instead, because eggs slip between the gaps out of reach until the adults are removed. That is a breeding-tank floor, not a general recommendation: it traps waste badly and is hard to clean, so it belongs in a short-term spawning tank that gets stripped down afterwards, never in the main display.
Layout That Protects the Swimming Run
Keep the middle of the tank open. Plant taller stems in the substrate along the back and sides, where roots need depth, and keep the front and centre as clear floor so the school has an unbroken run. Danios judge a tank by its length, and hardscape piled across the middle shortens the course they use all day.
Rinse gravel or sand thoroughly before adding it, lay it deep enough at the back for rooting, and keep it shallower at the front where vacuuming happens most. Add the floor before the water whenever possible, since tipping new substrate into a stocked tank clouds the water and panics a school that reacts to every sudden change.
Cleaning Under a Busy School
Danios eat quickly and messily, so vacuum the front floor with each water change, working in short passes while the school investigates the tube. Fine gravel needs a light plunge-and-lift to pull crumbs from between grains; sand needs a hover just above the surface. Check the corners behind plants where flow slows, because that is where the school's leftovers settle.
If the floor ever needs partial replacement, do a section at a time rather than stripping the whole tank. Beneficial bacteria live in the substrate as well as the filter, and a full change in one day can unsettle water quality in a tank stocked with an active, well-fed school.
Keeping the Record Straight
Because the danios themselves are undemanding, the substrate decision is often forgotten until someone buys a random bag for a repair. Noting the grain, colour, and depth once prevents mismatched patches and keeps cleaning instructions accurate for anyone else who maintains the tank. KinsLe Family Sharing keeps that floor record in the shared tank profile, so every family member who vacuums, replants, or tops up this danio tank works from the same setup the school started with.