What Is an Aquarium Powerhead? Do I Need One for My Fish Tank?
A powerhead is a device that looks like a little internal filter but has no filter material in it, and initially you might think, 'What use is this puny device to me?' and discard it in disgust. But wait! A powerhead is actually quite useful in some tanks; in others, it is almost essential.
What Does the Powerhead Do?
Let me set the scene for you: a four-foot tank, running an external filter. It's winter, so the ambient room temperature hovers around 18 degrees—or even lower sometimes. Far too cold for tropical fish. You have three heaters in the tank, but it never seems to get warmer than 22–23 degrees, no matter how high the heaters are set. What's wrong?
I'll tell you what's wrong: Regardless of how large your canister filter is, the flow is unlikely to create significant current in the tank. Water moves sluggishly, so your heaters heat the water immediately around them, then switch off, creating a pool of warmth in one area of the tank and leaving the rest of it frigid.
The Powerhead Keeps Water Moving
A powerhead will create a nice underwater current, keeping the water moving around your tank and causing your heaters to actually heat all the water. Not only that, but the current will give your fish something to swim against, which is appreciated by active river fish that otherwise will spend their lives wondering how they ended up in a small glass pond.
Of course, some fish don't like a great deal of current, but most fish robust enough to live in a large tank environment appreciate some current to frolic and play in. More importantly, the flow of water means that your water maintains a homogeneous temperature throughout the tank without developing weird cold and hot spots that will confuse the fish and potentially make them ill.
When Don't I Need a Powerhead?
- You don't need a powerhead if your tank is already using an internal filter with sufficient flow (you'll know if the flow is sufficient by placing a thermometer at the end of the tank away from the heater and seeing if it is at the same temperature as the water nearer the heater).
- You don't need a powerhead if your tank is under two feet. Probably.
- You don't need a powerhead if your external filter provides strong enough outflow to create a substantial current. I use the Eheim series of canister filters, which work well in most respects but which have a rather gentle flow that creates very little current at all.