Cold-Water River Tanks: Do You Need a Heater?

A lot of the fish that do best in river tanks come from cool mountain streams, not warm tropical swamps. That raises a good question as fall turns to winter: does a river tank need a heater?

The answer depends on your fish and your house.

Fish that like it cool

These fish come from cooler streams and are often happy at room temperature in a heated home:

• White Cloud Mountain minnows. See White Cloud care.
• Many hillstream loaches, which like cool, oxygen-rich water. See hillstream loach care.
• Some stream gobies.
• Native North American fish, where legal to keep.
• Goldfish (though they get big and messy for most river tanks).

When you might not need a heater

If your house stays in the mid to upper 60s °F all winter and the tank isn't near a drafty window, door or outside wall, a cold-water tank can often run without one. Many keepers do just fine.

When a heater helps

A heater is good insurance when:

• Your house gets cold at night or when you're away, or you turn the thermostat way down.
• The tank is near a window, an exterior wall or a door.
• You keep tropical fish too.
• Temperatures swing. Sudden drops are harder on fish than steady cool water.

Set the heater to the low end of what your fish like. It only turns on when the water drops below that, keeping things steady.

How to size a heater

A common rule of thumb is about 3 to 5 watts per gallon, with more for a cold room or a big temperature gap. Two smaller heaters at each end of a long tank spread the heat evenly and give you a backup if one fails.

In a strong-flow river tank, place the heater where water moves past it so the heat spreads.

Heater safety

• Use a heater guard to protect fish and the glass.
• Unplug the heater before taking it out of the water; a hot heater can crack when it hits air.
• Don't trust the dial alone. Check the real water temperature with a separate thermometer. See best aquarium thermometers.
• Watch for stuck heaters. A heater that sticks "on" can cook a tank. A separate controller adds protection.

The bigger risk: summer heat

For cool-water fish like hillstream loaches, the bigger danger is often summer, not winter. Water above the upper 70s °F holds less oxygen, and some cool-water species struggle. Keep tanks out of direct sun, add surface agitation, and use a fan across the surface on hot days.

Power outages

In winter, a power outage can drop a tank's temperature fast. See keeping an aquarium warm in a power outage.

Do fish hibernate?

Some fish slow down in cold water, but aquarium fish shouldn't be chilled to that point. See do fish hibernate?.

The bottom line

For a true cool-water river tank in a heated house, you may not need a heater. But a small heater set low is cheap insurance against cold nights and outages. A good thermometer is a must either way.

Related: How to Cycle a New Aquarium (Fishless Cycle, Step by Step)