Fish in winter: heat loss, drafts and evaporation

Quick answer

Dry indoor air does not dry out a fish. What a cold climate and a heated house actually do to an aquarium is different and less obvious: they pull heat out of the tank faster than the heater can put it back, and they evaporate water out of the tank far quicker than in summer.

Both effects arrive together in winter, and both are managed by the same weekly routine. The fish never leaves the water. Everything you change happens to the room, the heater, and the top-up jug.

• A tank in a cold room is only as stable as its heater, and heaters are sized for a temperature difference, not for a tank volume alone.
• Central heating that cycles on and off swings the room by several degrees, and a small tank follows the room closely.
• Evaporation concentrates everything dissolved in the tank, so the water gets harder and saltier as the level drops.
• Top up evaporation with dechlorinated fresh water, never with more of the same salted or mineral-adjusted water you used for a water change.
• A window sill, an exterior wall and a door to a cold hallway are the three worst places for a tank in winter.

Decide in 60 seconds

Why winter changes an aquarium

A heater does not heat water to a set temperature by magic. It adds a fixed amount of energy per hour, and the tank loses energy to the room continuously through the glass and the surface. The temperature settles where those two balance.

When the room drops, the loss side of that equation grows. A heater that held the tank comfortably when the room sat at a mild autumn temperature may run continuously in midwinter and still fall short. Nothing has broken. The job simply got harder.

Evaporation is the second half of it. Heated indoor air in a cold climate is dry air, because cold outdoor air holds little moisture and warming it lowers its relative humidity further. Dry air over a warm water surface takes water up fast, and an open-topped tank can lose a visible amount in a week.

That evaporation is where the real chemistry problem lives. Only the water leaves. Everything dissolved in it stays, so hardness, conductivity, any aquarium salt you have added, and nitrate all become more concentrated as the level falls. A tank that was correct in November drifts steadily upward through the winter if it is topped up carelessly or not at all.

Evaporation also cools. The energy to turn water into vapour is taken from the tank, so a briskly evaporating open tank is fighting its own heater. A lid solves both halves at once.

What a stable winter tank looks like

The temperature reads the same in the morning and in the evening, on the coldest day and the mildest one.

The water line sits close to a mark you have put on the glass, because you top it up before it drops far, not after.

The fish behave as they did in summer. They hold their fins out rather than clamped against the body, they use the whole tank rather than hanging near the heater or at the surface, and they eat with the same interest.

Fins held open and away from the body, normal colour, and active use of the whole tank. This is what you are protecting through the winter.

Coldwater species are not exempt. A goldfish is comfortable at cooler temperatures than a tropical fish, but it is no more tolerant of a rapid swing, and a goldfish tank on a cold window sill swings just as much as a tropical one.

Getting the room and the tank right

Move the tank away from the cold surfaces.

An exterior wall, a window, a conservatory and the draft from a frequently opened front door all pull heat out continuously. An interior wall in a room that stays heated is the single most effective change you can make, and it costs nothing.

Put a lid on.

A cover cuts evaporation sharply, keeps the warm layer of air over the surface, and stops the room's draft scouring heat off the water. It also prevents jumping, which is a separate and common cause of death.

Insulate the back and the sides.

A sheet of insulating foam board against the back glass, out of sight, reduces heat loss without affecting the view. This is standard practice in unheated garages and outbuildings and works just as well in a cold room.

Check that the heater is up to the job.

If the heater indicator light is on almost permanently and the temperature still sits low, the heater is undersized for this room in this season rather than faulty. Ask an aquatic retailer or an aquatic vet to size a replacement against your tank volume and your actual coldest room temperature. Some keepers run two smaller heaters at opposite ends rather than one large one, which gives even heat and means a single failure is not total.

Trust an independent thermometer, not the dial.

Heater dials drift. A separate thermometer, read at the same time each day, is what tells you the truth. A second thermometer at the other end of the tank tells you whether the water is actually mixing.

Keep the water moving.

A pump or air stone that keeps the surface turning over prevents a warm layer sitting at the top and a cold layer at the bottom, and it maintains gas exchange when the fish are working harder.

Topping up, and why it is not a water change

Evaporation and a water change are opposite operations and they need different water.

When water evaporates, only pure water leaves. To put the tank back where it was, you replace it with pure water: dechlorinated tap water or RO water at the same temperature as the tank, with nothing added.

When you do a water change, you are removing water plus everything dissolved in it, so the replacement carries the usual dechlorinator and any hardness or salt adjustment your setup requires.

Confusing the two is how tanks slowly become too hard or too salty over a winter. Topping up evaporation with salted water adds salt without ever removing any, and it compounds week on week.

Mark the correct water line on the outside of the glass with a small piece of tape. Then topping up is a glance rather than a judgement.

Add top-up water slowly, and pre-match its temperature. Pouring cool water straight onto a fish is the exact temperature shock you have spent the winter avoiding.

Changes that mean act today

Gasping at the surface, or crowding around the filter outflow.

This is an oxygen problem. Increase surface agitation immediately, check the temperature is not too high, and do not feed until it resolves.

Clamped fins, sitting on the bottom, and hiding after a cold night.

Chilling stress. Bring the temperature back to normal slowly rather than in one jump, and watch closely over the next week for secondary infection.

White pinhead spots appearing several days after a temperature drop.

Ich follows cold stress with a characteristic lag. Treat it as ich and act rather than waiting to see whether it spreads.

Frayed or reddened fin edges, cloudy patches on the body.

Bacterial and fungal infections take hold in stressed, chilled fish. These need identification rather than a broad-spectrum guess.

Water noticeably too warm, fish at the surface, heater light stuck on.

Unplug the heater at once. A stuck-on heater cooks a tank quickly and is far more dangerous than a failed-off one.

The routine that catches problems early

Read the thermometer every day, at the same time, and note anything unusual. It takes seconds and it is the only early warning you have.

Look at the water line every time you feed, and top up before it drops far rather than in one large correction.

Feel the heater cable and plug are dry and the equipment is above any possible splash line.

Once a week, test the parameters you normally track and compare them against the previous weeks. A steady upward creep across the winter usually means evaporation is not being replaced properly.

Once a month through the heating season, check the room itself. A radiator turned off in a rarely used room, a window left ajar, or a timer change on the household heating will show up in the tank before you notice it yourself.

What never to do

Do not pour hot water, kettle water or boiled water into the tank to raise the temperature.

Do not top up evaporation with salted, buffered or mineral-adjusted water. That is water change water and it belongs only in a water change.

Do not let the level fall until the filter intake sucks air. By then the flow, the filtration and the oxygenation have all already dropped.

Do not put a tank in an unheated conservatory, porch or garage for the winter without insulating and heating it deliberately.

Do not use a household space heater aimed at the tank glass. Uneven heating cracks glass and creates a hot spot the fish cannot escape.

Do not raise the temperature to "boost" a fish that looks unwell without knowing why. Warmer water holds less oxygen and speeds up some pathogens as well as the immune response.

Do not assume a coldwater tank needs no attention. It needs the same stability, just around a different number.

When to get help

Contact an aquatic veterinarian or an experienced aquatic specialist promptly if fish are gasping at the surface, sitting on the bottom with clamped fins, or developing spots, patches or fin damage after a temperature event.

Get help urgently if several fish decline at once, because a shared environmental cause moves faster than an individual illness and the whole tank is exposed to it.

A steady winter tank looks exactly like a steady summer tank. That is the whole objective.

Bring your temperature log, your recent water test results, the tank volume, the heater wattage, and how far the level had dropped before you last topped up. Environmental problems are diagnosed from that history far more often than from looking at the fish.