Why Is My Tank Cold in Winter? Check the Heater Before You Blame the Window

The first cold night of fall comes, the furnace hasn't caught up yet, and in the morning your thermometer reads a couple of degrees lower than it did all summer. The heater light is on and the water still won't warm up. A drafty window can be part of the problem, but it's usually not the main one. The real cause is a cold room, and a heater that's too small or in the wrong spot makes it worse. Here's how to tell which problem you have and how to fix it before the holidays.

Short answer: yes, a window can do it, but check the heater first

Fishkeeping guides tell you to keep tanks away from windows. Practical Fishkeeping warns that open windows cause draughts, and draughts bring temperature changes. So a window can matter. In most cases, though, it matters because it makes the whole room colder. It isn't usually blowing cold air straight onto the glass.

Winter shows you whether your heater can keep up. Most tropical freshwater fish do best around 78–80 °F, and if your house is cooler than that, the heater is all that holds the water there. A heater that worked fine at 72 °F in September can fall behind when the room drops into the 60s, or lower at night.

To tell the causes apart, watch two things:

• When it happens. If the tank is cold every morning but recovers by afternoon, the heater can't keep up with the night-time room. That points to heater size.
• Where it happens. If one end of the tank reads colder than the other, the heater isn't getting heat around the tank. That points to heater placement, or to a cold spot like a window or door on that side.

Why a winter room pulls the tank cold

The gap between the water and the air is bigger than most people expect. JBL notes that in winter a room can fall to 12–14 °C (about 54–57 °F) at night, while tanks usually run at 23–28 °C (about 73–82 °F). A heater facing that gap may run nearly nonstop and still lose ground.

Where the tank sits changes the air around it. LiveAquaria says heating and air-conditioning vents and exterior entrances can drastically alter the temperature near a tank, and that stable water temperature is imperative to fish health. Practical Fishkeeping puts windows on the same list. Aquarium Co-Op adds sunny spots, basements and places near air conditioning. It also notes that on a rack, the lower tanks tend to run cooler.

Then there's the top of the tank. Open water gives off heat all the time. JBL says a cover cuts that surface heat loss and keeps the light's heat in the tank. The common heater-sizing rules assume you have a lid. Aquarium Co-Op's 5-watts-per-gallon figure is based on a covered tank. If your tank is open on top, a heater sized by the rule starts out behind.

JBL also says that once the room falls below about 20 °C (68 °F), cold air coming in from outside can overwhelm the heater. That's the point where a drafty window or a door that's opened all day starts to make a real difference.

Is it the window? A quick check

If you can move the tank, the best spot is against an interior wall, away from windows, exterior doors and vents. Practical Fishkeeping also says to avoid kitchens, working garages and bathrooms, because their temperatures swing.

Most of us can't move a full tank on a whim, so test it first:

1. **Get a second thermometer.** Keep one near the heater and put the other at the far end. Aquarium Co-Op suggests putting a thermometer in the corner opposite the heater to confirm the heat actually reaches the other side.
2. Read both first thing in the morning, when the room is coldest, and again in the afternoon.
3. Compare. If both ends read low by the same amount, the whole tank is losing to the room, so look at heater size. If the end nearest the window or door is the cold one, that spot is pulling heat out faster than the water can carry it over.

Remember that a window works both ways. Direct sun through the glass can push the temperature up instead of down. LiveAquaria says sunlight can raise water temperature and encourage algae, and Practical Fishkeeping warns about overheating and algae from window sun. A tank by a south-facing window can be warm on a bright afternoon and cold by dawn. That kind of daily swing is a good reason to move it.

Is your heater big enough?

Sizing rules differ a bit depending on who you ask:

• Aquarium Co-Op: 5 watts per gallon to raise the water 10 degrees (°F) above room temperature, with a lid on. Its example is a 29-gallon tank on a 100-watt heater.
• Top Shelf Aquatics: plan on about 3–5 watts per gallon, and go higher when the gap between room and target temperature is large.
• JBL: 1 watt per litre, which works out to about 3.8 watts per gallon. Its lower figure of 0.5 watts per litre only applies if the room stays above about 20 °C (68 °F).

Here's a 40-gallon tank as an example. Top Shelf Aquatics puts it at 150–200 watts. At Aquarium Co-Op's 5 watts per gallon, it needs 200 watts to hold a 10-degree rise with a lid. A 100-watt heater bought when the room was warmer will lose ground once winter nights arrive.

A colder room asks more of the heater. Aquarium Co-Op's cold-room example is a 65 °F house where the water needs to come up 15 degrees, and its advice there is to add a second heater. When a tank needs 200 watts, Co-Op recommends two 100-watt heaters over one 200-watt unit. Top Shelf Aquatics also suggests two smaller heaters for tanks over about 40–50 gallons. For my old 55-gallon long, a heater at each end is the setup I'd use.

Is one bigger heater ever the answer? JBL says a larger heater doesn't raise your yearly power use, because it reaches temperature faster and then shuts off. But a bigger heater is longer and often won't fit, and JBL calls it an emergency fix only. Two heaters spread the heat better, and if one fails you still have the other.

Place the heater where water moves

A heater only warms the water that touches it. Moving water is what spreads that heat through the tank. Aquarium Co-Op says to put the heater right next to the filter output or pump for maximum flow. It also says to mount a long tube heater at about a 45-degree angle.

In a river tank, that means putting the heater near the flow. My RTK-XL pumped water up on the left side, carried it across and cascaded it down through three pools. In a setup like that, the heater should sit where the pump pulls or pushes water, not tucked behind driftwood in a quiet corner.

The reason is the thermostat. JBL explains that when the water around a heater doesn't circulate, the heater warms its own pocket, decides the tank is up to temperature and shuts off, while the rest of the tank can be 3–4 °C (roughly 5–7 °F) colder. A thermometer next to the heater will show that same warm pocket and tell you everything's fine.

That's why the second thermometer goes at the far end, as far from the heater as you can get it. If it reads several degrees lower than the heater end, fix the placement or the flow before buying a bigger heater.

Cheap fixes and safety

Before Thanksgiving brings the real cold, a few inexpensive changes help:

• Put a lid on it. A cover cuts surface heat loss and keeps the light's heat in, and the wattage rules assume one anyway. I learned early to swap my light kit for a full screen lid, because crabs and some reptiles get out of almost anything. A lid is worth having on a mixed tank for that reason alone.
• Insulate the back and sides. JBL calls polystyrene or a foam pad against the outer walls a cheap, easy fix. The back and the side facing a window are the obvious places to start.
• Insulate the window. Weatherstripping or window film lowers the draft and the room's overall heat loss, which helps the tank too.
• Skip undergravel heating as your main heat source. JBL says it isn't suitable for heating the water.

Then there's safety. Two smaller heaters also protect you when one fails. Aquarium Co-Op points out that one small heater that overheats is unlikely to overheat the whole tank on its own, and if one dies, the other keeps the water from crashing. If a heater ever sticks on and the temperature keeps climbing, unplug it right away and replace it. Replace one that can't hold temperature too, even after you've fixed placement, flow and the lid. A heater that's losing ground in October will likely do worse in January.

Sources (5)

• JBL: Aquarium heating in winter
• Aquarium Co-Op: What size heater do I need?
• Practical Fishkeeping: Diploma week 5, day 1
• LiveAquaria: Aquarium placement
• Top Shelf Aquatics: Aquarium heater size calculator

Photo by joshuaclifford123 on Pixabay.