The short answer
Corydoras don’t strictly need sand to survive, but they strongly prefer it — and fine, smooth sand is genuinely better for their health. Cories feed by sifting the substrate with delicate whisker-like barbels around the mouth. On sharp or coarse gravel those barbels get worn down and can become infected, leaving the fish unable to forage properly. Smooth sand lets them do exactly what they’re built to do.
Why sand suits them
In the wild, corydoras cruise sandy riverbeds, taking mouthfuls of substrate and sifting food out through their gills. Fine sand:
- Protects the barbels they rely on to find food
- Lets them sift naturally, which is both feeding and enrichment
- Keeps leftover food on the surface where they can reach it, rather than buried in gravel
On sharp gravel you often see cories with eroded or missing barbels — a clear sign the substrate is too harsh for them.
If you already have gravel
You don’t have to rip everything out overnight, but aim to move toward a fine, smooth, rounded substrate. If you keep gravel, choose the smallest, smoothest grains you can and keep it spotlessly clean. A shallow layer of inert sand over part of the tank gives cories somewhere proper to forage.
Whatever the substrate, keep a sand layer shallow and stir it gently at water changes so gas pockets don’t form.
The bigger picture
Substrate is one part of good cory care — they’re social, bottom-dwelling shoalers that need a group and cool-to-tropical, clean water. Read the full bronze corydoras care guide for tank size, numbers and diet, and see our sinking food picks to make sure your cories actually get their share.