Best Substrate for a River Tank: Sand, Pebbles, or River Rock

Look at a real stream bed and you'll see the plan for a river tank: fine sand where the water slows down, gravel and pebbles where it moves, and smooth rocks where the current is strongest. Copying that is what makes a river tank look real, and it's what keeps bottom-dwelling fish happy.

Your options

Sand - Natural for slow-water areas and for fish that sift or bury (many loaches, assassin snails). - Easy on the barbels and bellies of bottom dwellers. - Too fine a sand can blow around in strong current. Use it in calmer zones or behind rocks.

Fine to medium gravel - Stays put in moderate current and holds plant roots. - Choose smooth, rounded gravel, not sharp, crushed stone.

Pebbles and river rock - The signature river-tank look. Mix sizes, from marble size to fist size. - Stays in place in strong flow. Smooth rocks are where hillstream loaches cling and graze. - Leave gaps that make caves and hiding spots.

A natural layering plan

1. Base: fine gravel or a sand-gravel mix across the bottom.
2. Current zone: pebbles and rounded river rock, larger toward where the flow is strongest.
3. Slow zone: a patch of sand behind larger rocks or driftwood, where debris settles.
4. Accent rocks: a few large smooth stones as landmarks.

Safety notes

• Rinse everything before it goes in the tank.
• Avoid rocks that change water chemistry unless you want harder water: limestone and many seashells raise pH and hardness. A drop of vinegar that fizzes on a rock is a sign it contains carbonate.
• Collected rocks: scrub and boil or bake them, and avoid rocks from polluted areas. Check local rules on collecting.
• Keep big rocks stable, resting on the tank floor, not perched on substrate fish can dig out.

Cleaning

Debris collects in the slow zones; that's normal in a river too. Vacuum those spots during water changes. See also cleaning aquarium rocks.

Get the bottom right and the rest of the river tank (flow, plants and fish) falls into place. See the river tank owner's guide.