Aquariums and Wet Windows

Every litre a tank loses to evaporation ends up in the room’s air, and in winter that air delivers it to the coldest glass in the house. Whether that matters depends mostly on tank size, the lid, and how well the room is ventilated.

When it matters

• A pet-care guide from Planète Pets calls the effect negligible for a small, well-covered 60-litre tank. A large open tank of 200 litres or more in a bedroom or poorly ventilated room can noticeably raise humidity and, over time, feed mould on cold surfaces.
• Heated tanks (24–26 °C water) evaporate continuously, and dry winter air speeds it up. Many fishkeepers top up more often in winter even though the room is cooler.
• Airtight modern homes hold that moisture in. Older, draftier homes vent more of it on their own.

What to do

1. Cover the tank. A fitted glass or acrylic lid is the biggest single step. One aquarium guide estimates it cuts evaporation by roughly 70–90%, because humid air stays trapped above the water and condenses back into the tank.
2. Vent the lid a little if it drips or fogs heavily. Propping it 5–10 mm along the back keeps gas exchange without opening the whole surface.
3. Calm the surface where your fish allow it. Spray bars, air stones, and fans blowing across the water all increase evaporation.
4. Don’t run the tank warmer than the species needs, and check that the heater isn’t overshooting.
5. Air the room daily, even for ten minutes, and run a dehumidifier in small or windowless rooms with big tanks.
6. Measure. Keep a hygrometer in the tank room. If it reads well above the rest of the house or above about 60%, the tank is a real moisture source.

Note: aquarium sites often suggest a humidifier in winter to slow evaporation. That works against dry windows, so prefer a lid.

Sources: Planète Pets, Aquarium and room humidity; App-aquatic, Aquarium water evaporation; Gallery Aquatica, Lid condensation in winter; Real Aquatics.

Originally published at https://dry-glass-notes.surge.sh/articles/aquariums-and-window-condensation.html (AI-assisted, sources cited). If this helped, zaps go to hollisvarrow3753@coinos.io.