Why the hole in the ice matters more than the ice
Pond fish are built for cold water. As temperatures fall, their metabolism slows to a near standstill and they rest in the deepest part of the pond, where the water stays a few degrees above freezing all winter. What they cannot survive is a sealed pond. Under a continuous sheet of ice, the gases released by rotting leaves and the fish's own waste have nowhere to go, while fresh oxygen cannot get in, and the water quietly turns hostile beneath a surface that looks perfectly peaceful. Keeping one modest hole open in the ice — for gas exchange in both directions — is the single winter job that everything else supports.
Keep the hole open the gentle way
A floating pond de-icer holds a hole open with the least effort and the least disturbance, warming just enough water around itself to keep a breathing gap in the ice. A small air pump with an air stone set shallow does the same job by movement, and has the advantage of adding gentle circulation near the surface. What you must never do is smash or chop the ice to make a hole: the shock waves from each blow travel through the water and can stun or kill fish resting below. If a hole has frozen over and you have no de-icer, rest a pan of hot water on the ice and let it melt its way through slowly.
Stop feeding before the freeze arrives
Cold water changes what a fish's body can do with food. As the pond cools toward the point where the fish stop rising and showing interest, digestion slows with everything else, and uneaten food sinks to rot on the bottom, feeding exactly the gas problem the ice hole is fighting. Take your cue from the fish: when they stop coming up eagerly, stop offering. A short period of commercially made cool-water food is fine while they are still active in autumn, but the winter itself is a fast, and healthy pond fish carry the reserves to manage it. Remove any food that goes uneaten within minutes rather than leaving it to decay.
Prepare the pond while the water is still open
The work that decides the winter happens weeks before the first frost. Net the pond in autumn so falling leaves land on the net instead of in the water, and lift out any leaves and dead plant matter that have already sunk, because that material rotting under ice is the main source of the trapped gases. Trim back dying marginal plants, check that the deepest zone is genuinely deep — the fish need a layer of water at the bottom that never freezes — and confirm the de-icer and pump work before you need them. If your waterfall or fountain would freeze solid or splash water onto the ice, shut it down for the season and store the pump as its maker directs.
During the freeze, and as the thaw comes
Midwinter care is mostly watching: is the hole still open, is the de-icer still floating level and working, has heavy snow buried the pond so deeply that light no longer reaches the water? Brush snow off a section of ice after big falls. Do not feed, do not net fish, and do not disturb the bottom. At the thaw, the danger flips — warming water wakes bacteria before it wakes the fish's defences, so restart feeding later and more gradually than your impatience suggests, and watch for fish that hang listlessly or show fuzzy patches. Those winter rounds are easier kept faithfully when they sit in KinsLe as Care Teams duties, assigned to whoever is home that week and marked complete with a timestamp.