Powerhead, gyre, oscillator or sweep: which one actually reaches the back of the tank?
Every reef keeper finds the same thing eventually. A pile of detritus sitting undisturbed behind the rockwork, in the one place the flow never seems to get to. So you turn the pump up. The sand at the front starts lifting, the corals near the outlet get battered, and the grey-brown pile behind the rock stays exactly where it was.
Most people call that a dead spot. It isn't actually dead. The water there is moving, just not fast enough to shift anything, and that difference is what this whole comparison turns on. There are four broad ways to deal with it, none of them best at everything, and which one suits you depends on what your tank actually looks like. Here is how they compare, including the parts that do not favour what I sell.
Water gets everywhere. Fast water doesn't.
One thing first: in a tank with a decent pump, water circulates through every part of it. That is not in question, and a fixed powerhead does that job perfectly well.
What varies is how fast the water is moving at each point: its velocity. A pump pushes out a jet. The water in the core of that jet moves fastest, it slows as it spreads out and travels away from the outlet, and it slows again where it has to find its way round rock. So every tank has a pattern. A few areas sit in fast water, most sit in moderate water, and some only ever get slow water.
Detritus does not care whether water reaches a spot. It cares how fast that water is moving. Where the velocity drops below what it takes to keep particles moving, they settle, and that is your dead spot. It is really a slow spot, and it stays slow because the jet takes the same path through the tank every time.
That is also why turning the pump up does not fix it. More power makes the fast water faster and the slow water a little less slow, but the pattern stays where it is, and the areas at the bottom of the scale are still at the bottom. Pulse and random modes vary how hard the jet runs over time, which helps, but a pump that stays in one place and faces one way still sends its fastest water down the same path.
A torch, with one correction
Think of a pump as a torch. Switch it on and it throws a beam in one direction. Everything in the beam is brightly lit, and everything outside it gets less.
The comparison breaks in one useful place. Put a book on a shelf and the torch leaves a shadow behind it. Water is not like light: it does get round the book. It just arrives slowly, having lost its speed on the way. So when reefers talk about a flow shadow, what it really means is a patch where the water is always slow, not a patch where it never goes.
Most flow arguments are about the beam: make it wider, make it stronger, make it pulse. You can only widen a beam so far, though, and the slow patches sit where they do because of where the beam starts.
The fixed powerhead
This is what most of us start with and it is genuinely good kit. Strong, controllable, cheap to run, and you can aim it precisely at the SPS cluster that wants it.
What it cannot do is move. In one spot it produces one flow pattern, and in a tank with open rockwork and plenty of gaps that is often all you need. On a packed aquascape, the pump is usually fine and the position is the problem.
The two normal fixes are a second pump, or moving the first one every few weeks. Both work. One costs money and another cable, the other costs your patience.
The gyre
A gyre takes the beam and stretches it sideways, so you get a wide sheet of flow running along the tank rather than a cone. It is an elegant piece of engineering, and on a long, shallow, open-scaped tank it produces lovely even movement.
The limitation is the same one, widened. The gyre still sits in one place, so anything shielded from that sheet stays in slow water, including a cave, the far side of a rock stack, and the water directly behind the pump's own mounting position.
The oscillating mount
The third option swings the pump through an arc. The pump sits on a rotating or pivoting mount and the beam sweeps side to side, so fast water passes over a wider area than a fixed pump manages.
Back to the torch. This is standing in one place and waving the torch about. The lit area is bigger and it changes through the arc, which is a real improvement on a beam that never moves at all.
What stays the same is where the beam comes from. The origin is still one fixed point on the mounting plate, so anything tucked directly behind a rock relative to that point is behind it at every angle of the arc. The slow patch narrows and shifts rather than going away. That is the difference between changing where a beam is aimed and changing where it starts.
The swept rail
The fourth approach changes the question. Rather than making one beam cover more ground, it moves the source: the powerhead you already own rides a carriage along a rail and travels front to back, so the flow arrives from a different place every few seconds.
That matters because of how the slow patches form. With a fixed pump they never move, since the fastest water always takes the same path. Move the origin and the path of the jet moves with it. Areas that would always sit in slow water take their turn in the fast water, and over one sweep the same rock face sees flow from the front, from an angle and from the side. The water was always getting there. Now the fast water gets there too.
Fusion Flow Linear is the worked example. Four sizes, named by how far the carriage travels: the 230 for tanks about 340 to 440 mm front to back, then the 330, 430 and 470 for roughly 440 to 540, 540 to 600, and 600 mm and deeper. Prices start at £229. You teach the two end points by hand and set the speed on a dial, so both how far it goes and how fast are yours to choose. The rail is hardened steel and runs inside a housing with an overlapping cover and a drip edge, which keeps it dry above saltwater without any seal, and that geometry is the part that is Patent Pending, GB2619546.1.
There is no extra pump and no extra plug, because it moves the pump you have already wired in.
Side by side
| What you want | Best answer |
|---|---|
| Fast water reaching every part of a packed aquascape | The swept rail, because the fast water moves with the source rather than depending on raw output |
| Even, gentle, wide movement on a long open tank | A gyre, and it is hard to beat at it |
| Wider coverage with the simplest upgrade from a fixed pump | An oscillating mount |
| Strong flow on one specific coral, all the time | A fixed powerhead, every time |
| Nothing new to wire in | The rail, since it carries a pump you already run |
| Simplest possible install | A powerhead. A gyre is easy too. The rail is the one that asks you to measure the tank first |
| Lowest price | A powerhead, and by a distance |
Where a sweep is the wrong buy
A slim rail sits across the top of the tank. That is the price of admission and it is a real change to the look of a rimless, open-top setup. If the clean line over the water matters to you more than what is happening behind the rock, buy a second powerhead instead. I would rather say so here than have it arrive and disappoint you.
It is not a high-flow specialist. If you want one SPS wall in fast water all the time, keep a dedicated pump pointed at it. A sweep gives every area its turn in the fast water. A fixed pump gives one area fast water constantly, and sometimes that is exactly what a coral wants.
It has moving parts above your tank, and they need servicing. A belt, a carriage and a rail that wants regreasing periodically. A powerhead has one moving part and you clean it twice a year. That is a fair point against the rail. What I will say is that the firmware is written to keep the carriage moving: if you pause it and forget, it restarts itself after ten minutes, because a pump that stops in one place brings back exactly the slow patches it is there to move.
It needs clearance. Room above the water for the rail, clear rim at the back for the clamp, and about 65 mm behind the tank. If you run a mesh cover, the Feed Port Pro is a 75 mm feeding port that lets you feed through the netting instead of lifting it off every day.
How to choose
Three questions, in this order.
How deep is the tank, front to back, inside the glass? That sets which sweep fits, and it is the only measurement that matters. The sizing guide walks through it.
Is the aquascape packed or open? Open rock with plenty of gaps lets fast water through on its own and rarely needs a moving source. A packed stack with caves is exactly where one earns its money.
Does a rail across the top bother you more than the detritus in the corner? If yes, buy a powerhead and be happy. There is no shame in the answer being the cheap one.
The short version
Water gets everywhere. Fast water goes where the pump sends it. A powerhead throws a torch beam. A gyre widens it. An oscillating mount waves it about. A swept rail moves where the torch is standing. The first three leave the same slow patches in the same places, however hard you run the pump. The fourth moves them.
If you want to know which size suits your tank, send me the model and the measurement front to back inside the glass. I answer these myself.
