What You Must Know About Indoor Fish Keeping: The Truth and Misconceptions of Heater Rods for Cold-Water Fish

Cold-water fish don’t necessarily need heater rods. The conclusion here is: generally, when keeping fish indoors, a heater rod is needed, while outdoors, it is not.

Some might argue, “That’s not right! Many fish keepers don’t use them, the fish sellers told me it’s unnecessary, are you trying to deceive me?”

In fact, I’m not deceiving you; it’s the fish sellers who are misleading you. If they told you that cold-water fish also need heater rods, would you still buy from them? Isn’t the purpose of keeping cold-water fish to save on electricity and effort? Which salesperson would honestly admit this? Please don’t rush to argue just yet; I’ll explain why in a moment.

The Needs of Indoor Fish Keeping

First, let’s clarify that I’ve always emphasized that the role of the heater rod is to maintain a constant water temperature, with heating being secondary. Cold-water or eurythermal fish are not afraid of the cold or heat, but they are most vulnerable to sudden temperature fluctuations and repeated changes in water temperature.

Fish are poikilothermic animals, meaning their body temperature changes with the surrounding environment. For example, the morning and evening temperatures might be fifteen degrees Celsius, while at noon it could reach twenty-five degrees. With such a large temperature difference, you would need to put on or take off a layer of clothing. But how should fish cope?

The Impact of Water Temperature Fluctuations

In indoor fish keeping, the water body is usually small, making temperature fluctuations very noticeable, hence the need for a heater rod. Even if you keep cold-water fish and set the water temperature to eighteen degrees Celsius, you still need a heater rod to maintain a stable temperature.

Characteristics of Outdoor Fish Keeping

Outdoor fish keeping usually involves larger water bodies, with the smallest being a canvas pool. Although canvas pools are relatively small, they can hold hundreds of liters or even a ton of water, not to mention fish ponds, which are even larger and typically measured in tons—one thousand liters equals one ton. Therefore, outdoor water bodies are very large, and the temperature fluctuations are relatively smaller.

The Specific Heat Capacity of Water and Temperature Changes

Water has a very high specific heat capacity, meaning the more water there is, the slower the temperature changes. For example, boiling a kettle of water versus a large pot of water: the large pot heats up slower. This is why wild fish can survive in winter—the water temperature changes so slowly. One cubic meter of water weighs one ton, and one ton of water equals one thousand liters. River water is generally calculated by square kilometers, with a depth of about four to five meters. You can calculate it yourself. If the river water wants to increase or decrease by one degree, it may take several days, as water temperature changes very slowly. Therefore, the fish have long adapted to this environment.

Do you really think the fish sellers are experienced in fish keeping? Not necessarily.