It starts with how these fish eat
Neither of these fish feeds from the surface or the open water. A corydoras spends its day nose-down, pushing its snout into the substrate and taking up mouthfuls, which it sifts so the food stays and the grains fall away through its gills. Around its mouth are barbels — whisker-like feelers packed with taste buds — that do the searching, tasting the bed ahead of the fish like fingertips. Loaches work the same ground differently: a kuhli loach slides eel-like over and partly into the substrate, and many loaches dig, shuffle stones, and rest their full weight on the bottom. For both, the substrate is not scenery. It is the surface their most delicate anatomy presses against for hours every day.
What sharp gravel does to barbels and skin
Coarse gravel with cut or jagged edges abrades a corydoras every time it feeds. The barbels wear shorter, then ragged, and the abraded snout is open to the bacteria that live in any substrate, so erosion and infection follow; a cory with shortened, stubby barbels on sharp gravel is showing you the cause in plain sight. Loaches suffer the equivalent injury underneath: most are scaleless or nearly so, and a soft belly dragged over rough stone scrapes raw, while species that dig can tear skin forcing themselves under sharp pieces. Sand avoids both injuries because each grain is tiny and rounded: it moves aside as the fish pushes in, supports the barbel's search instead of resisting it, and cushions a resting belly.
Sand and fine gravel: choosing between them
Smooth play sand and purpose-made aquarium sands are the first choice for corydoras, whose sifting behaviour is at its most natural and entertaining over a soft bed they can plunge into up to their eyes. Very fine gravel, roughly the grade of coarse sand with visibly rounded grains, is an acceptable second: run a handful through your fingers, and if it feels gritty-smooth rather than crunchy or sharp, it will treat barbels kindly. What matters is the edge, not the label — some decorative gravels sold for aquariums are crushed and genuinely sharp. Avoid deep beds of fine sand compacting in a neglected tank; an inch or so, stirred gently at water changes, stays sweet.
Changing an existing tank over safely
If sharp gravel is already in the tank, do not strip it all out in one afternoon: much of the tank's beneficial bacteria live in that bed, and removing everything at once can unsettle the water. Swap a third of the floor at a time over a few weeks, pushing a divider of card or a row of stones between old and new, or lay sand over a lowered area to create a sifting patch the fish can use straight away while the change proceeds. Rinse new sand until the water runs clear before it goes in, or the tank will cloud for days. Watch the corydoras during the change: they will vote immediately, gathering on the sand side to feed.
The payoff you should see
On the right substrate, a corydoras group behaves differently within days — sifting in the open, barbels long and busy, resting in a row on the sand. A loach buries itself with only its head showing, which is contentment, not illness. Barbels that continue to shorten over a smooth bed point to a different problem, most often poor water quality in the layer just above the substrate, so review your recent maintenance before blaming the sand. KinsLe's Kinny can answer a follow-up question — he draws his answers from the KinsLe Answers library.