Why corners collect what the filter misses
Filter flow follows the tank's open lanes and dies in its corners, and everything the water carries — uneaten food, plant fragments, fish waste — settles exactly where the current gives up. In a corydoras tank the problem compounds: the fish spend their lives working the bottom, stirring fine debris loose from the middle and letting it migrate outward to the still edges. Left alone, a corner becomes a mulm pocket that clouds the water at every disturbance and feeds algae and ammonia out of proportion to its size.
Siphon the corners first, not last
Make the corners the opening act of every water change, while the water is still deep and your patience is fresh. Work the siphon slowly along each corner seam and behind the ornaments that shelter them, hovering the tube just above the substrate so the suction lifts settled waste without excavating. Corydoras sift sand for a living, so a violent gravel clean undoes their table as well as their toilet; a hovering pass takes the mulm and leaves the surface texture they work through. Corners done first also means corners actually done — they are the first casualty of a tidy that starts to run long. Between changes, a turkey baster is the corner's best friend: a few puffs along each seam lift settled waste into the water column where the filter can take it, a thirty-second job that stops a pocket forming in the first place.
Keep a gentle drift along the bottom
Between changes, the fix is circulation at floor level. Angle the filter return so part of its flow travels along the back glass and down into a corner rather than dying at the surface, or place wood and rock so they channel a slow drift across the bottom instead of damming it. The goal is not current the corys must fight — they are bottom fish, not fast-water loaches — but water that never fully stops, so debris keeps travelling toward the siphon spots and the filter intake instead of settling in the seams.
Feed where the cleanup happens
Scattered sinking food guarantees corner waste, because pellets the fish miss roll or get nosed into the still edges. Feed in one open patch of substrate, the same spot each time: the group learns the restaurant, eats thoroughly in the open, and what little remains sits exactly where your siphon already goes. A small feeding dish sunk flush with the sand makes the arrangement stronger still and gives the corners one less source of supply.
Watch what the corners tell you
A corner that re-fills within days of cleaning is information: something upstream — an over-generous feeding hand, a plant shedding leaves, a filter losing pace — is producing more than the routine removes. Fix the source rather than siphoning harder. KinsLe Care Teams can list corner-siphoning, feeding-spot tidying and the bottom-flow check as named steps within the tank's change-day duties, so the routine that keeps the seams clean is followed by whoever does the change, not just by the person who invented it.