Winter in India is not the same everywhere. In Mumbai, it is mild, but in Delhi, Punjab and the hills, nights can fall into the single digits. Fish do not care which city you live in. They care about how fast the temperature changes. A drop of 4 to 6 degrees overnight is enough to stress tropical fish and invite disease. Here is how to protect your tank.
Why Cold Nights Hurt Fish
Fish are cold-blooded, so their body temperature follows the water. When it drops, their metabolism slows, their immune system weakens, and they become easier prey for infections such as white spot (ich). The danger is less the cold itself and more a sudden swing, such as a fall of more than 2 C in a few hours. Our guide to what temperature your aquarium should be covers the basics.
Target Temperatures
|
Fish group |
Comfortable range |
|---|---|
|
Tropical community fish (tetras, guppies, rasboras, mollies) |
24 to 28 C |
|
Bettas |
25 to 28 C |
|
Goldfish |
18 to 24 C |
|
Neocaridina shrimp |
20 to 26 C |
Choosing a Heater
- Wattage: plan on roughly 0.5 to 1 watt per litre, depending on how cold your room gets. A 100 L tank might use 50 to 75 W in a mild winter and up to 100 W in a cold northern room.
- Two smaller heaters are safer than one big one on large tanks. If one fails, the other keeps the tank from dropping completely.
- Thermostat and guard: choose an adjustable, fully submersible heater and fit a guard so fish and shrimp cannot touch it.
- Placement: put it near the filter outlet so warmth spreads evenly.
- Verify with a separate thermometer. Never trust the heater dial alone.
- Never run a heater out of water. Unplug it before water changes.
Easy Protection Without Heavy Equipment
- Move the tank away from windows, doors, fans and AC vents.
- Insulate the back and sides with a foam or polystyrene sheet, leaving the front clear.
- Use a well-fitting lid to hold in heat.
- Do smaller water changes of 15 to 20 per cent, using temperature-matched water treated with Sunken Garden Anti-Chlorine.
- Feed a little less when the water is cool, because digestion slows.
Power Cuts in Winter
A large, insulated tank loses heat gradually, so keep the lid on and do not open the tank unnecessarily. A small battery-powered air pump keeps oxygen flowing if the filter stops. Restart the filter as soon as power returns.
Watch for Trouble
Fish sitting at the bottom with clamped fins, sluggish behaviour or tiny white spots can signal cold stress or ich. Raise the temperature slowly and use a suitable treatment as directed on the label.
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