Water Movement and Pumps

Oxygen. Surface movement is the main way oxygen gets into the water: moving water keeps bringing new water to the surface, where oxygen comes in from the air.

Into the coral. Every coral has a thin layer of still water on its surface, the boundary layer. It's thicker in low flow and thinner in high flow. A thick layer slows everything passing in and out: gases, nutrients and waste. Flow also delivers food, since no coral lives on light alone, and helps cool the coral's surface.

Waste and detritus. Flow keeps fish waste and leftover food moving, so it reaches the filters. Without it, waste settles into the rock and sand.

More light speeds up a coral's metabolism, so it needs more flow. Water movement is one of the most important parts of a system.

Turnover is the total flow per hour, counted in tank volumes. IMARCS uses at least 15 times the tank volume per hour as a minimum; 25 to 50 times of turbulent, non-laminar flow is better, depending on the species (Michael Maddox, President of IMARCS). Expect only 70 to 80% of a pump's rated flow once it's installed.

But turnover is a relatively poor way to measure flow, and it says little about what a coral feels. Surge devices, standing waves and similar methods can be much more effective than chasing a turnover number (Michael Maddox, President of IMARCS). A strong pump pushing water across the front glass does nothing for corals the water never touches. What counts is flow that reaches each coral.

More isn't always better. Small-polyp stony corals generally want more flow than large-polyp stony corals and many soft corals. A coral with flow on only one side can grow more on that side, and a dense colony can block flow to its own center.

Laminar flow is one narrow, steady stream in one direction. A simple powerhead running all the time makes mostly laminar flow. Aimed at one coral, it hits the same side all the time.

Turbulent flow mixes directions and changes. Random modes on DC pumps change speed and direction all the time; a reef crest mode, for example, changes every 1 to 3 seconds. The next sections show how to make turbulence, standing waves and pulses.

Two pumps on opposite walls. This is the minimum stable layout; a single pump leaves dead areas on the far side. Set them to alternate, so the flow keeps reversing; reverse every 30 to 90 seconds to keep detritus in suspension for the filters.

Aim at rock or glass first. Never aim a flow pump straight at a coral. Aim it so the flow hits rockwork or glass, then spreads across the corals.

Cross the flows. Two flows meeting break into mixed, turbulent water. Mix pump types: combining different pumps often works best. Use modes: random and pulse modes (section 7) and standing waves (section 6) add the changing motion a reef has. Section 5 shows how to push flows into each other.

Push flows into each other. Aim two pumps so their streams meet in open water, not at a coral. Where they meet, the water breaks into mixed, changing directions. Try different meeting points: the middle, above the rock, near the surface.

Use different angles and heights. Aim one pump up toward the surface for surface movement and another lower, across or under the rock. A gyre pump can even do both: one side angled toward the surface for agitation, the other side lower into the tank. Different pump types meeting, such as a gyre's wide sheet and a propeller pump's stream, mix the water in different ways.

Run pumps in opposition. Link DC pumps so one speeds up while the other slows, and the meeting point keeps moving. For example, run one pump at 50% and another at 5%, and swap them every 20 minutes.

Pulse. Each pulse pushes a mass of water out from the pump, and it spreads as it travels across the tank. Pulses from pumps at different places, or at different timings, keep changing where the water goes. A random mode does this on its own (section 7).

Change one thing at a time, then watch: how the corals' polyps and tissue move, where food drifts, and where detritus settles. If the sand starts blowing around, lower the speed or raise the angle.

Water in an enclosed basin can rock back and forth at its own natural rhythm, like water sloshing in a bathtub. This is a standing wave (a seiche). The rhythm depends on the basin's length and depth. The water rises and falls most at the ends; at the node, near the middle, the surface barely moves up and down.

IMARCS recommends standing waves wherever possible (Michael Maddox, President of IMARCS). They keep detritus suspended so the filters can remove it, and they move water through 100% of the aquarium, including behind and under rockwork.

One pump is enough. A single pulsing pump at one end can build a standing wave, so you don't need pumps on both sides of the tank. That's especially useful for peninsula aquariums (Michael Maddox, President of IMARCS).

How to make one: run a pump on a short timer, or use a DC pump's pulse mode. Pumps at opposite ends taking turns work too. Start with short on and off times, about a second each (a wave box example: 0.85 seconds on and 0.85 off), and adjust in small steps while watching the surface at the ends. When the timing matches the tank's own rhythm, each push adds to the last and the wave builds: the water at the ends rises and falls more and more. The right timing depends on the tank's length, depth and rockwork, so it's found by trial and error.

Watch it build the first time. A wave that grows too big can splash over the top or surge into the overflow; shorten the run time or lower the pump speed until it's steady. Check that the sand isn't being blown around.

Constant: one speed. Pulse: the pump speeds up and drops or stops in a rhythm. Pulse settings typically run from fractions of a second up to 30 seconds. Short pulses from one pump, or from pumps at opposite ends, can build a standing wave (section 6).

Common DC pump modes include: lagoon, gentle speed changes; reef crest, random speed changes with sudden surges like waves arriving; short pulse, ramping up quickly then stopping; tidal swell, a cycle over two hours ending in a surge that lifts detritus; gyre, wave-like flow with full stops; and nutrient transport, a wave phase followed by a strong surge to move detritus toward the overflow. A gyre pump's alternating mode reverses its flow direction on a cycle from 2 minutes to 12 hours.

Feed mode slows or stops the pumps for a few minutes; often for about three minutes. Night mode runs them slower at night; for example 50% for 10 hours, starting after lights off. Pumps can be linked to work together, for example in opposition so one speeds up while the other slows. After any program change, watch the tank for a while: the corals, the surface and the sand.

A wave controller (wave timer) switches AC powerheads on and off. One common type has two channels, each with a wattage limit, so pumps can alternate (one on while the other is off) or run synchronized (both together), with a dial for the interval, plus a feed program.

Pumps on opposite walls on alternating channels make the flow keep reversing. With short intervals, they can make a standing wave (section 6). Check each channel's wattage limit before connecting pumps, and keep the controller dry and off the floor (Module 2).

A submersible pump sits in the sump under water. It's simpler to set up and generally cheaper, but it adds its heat to the water. An external pump sits outside the sump, piped to it. Choosing the right pump can matter for heat: a pump that adds less heat can help avoid needing a chiller (Module 14).

An AC pump runs at one speed. Its flow can only be cut with a valve, which strains the pump and makes it louder. A DC pump has a controller that sets its speed, so flow can be matched to the drain. DC pumps generally use less power, run quieter and cooler, often have a feed mode, and many work with a battery backup. They tend to cost more. Fit an inlet screen on a DC return pump when it's used submersed, and never run it dry.

A powerhead is a small pump that sits in the tank and makes a narrow, steady, mostly laminar stream. A propeller pump moves a large volume of turbulent water without the strongly directed force of a powerhead. Inside, a motor turns a magnet, which spins the propeller; a cage keeps fingers and animals off the blades.

The pump body and its cord sit in the display, so place them where they can be reached and hidden. The cord needs a drip loop (Module 2). Mounts hold the pump to the glass; check they're firm during routine work.

A few pumps are magnetic coupled: the motor sits outside the glass and turns the propeller inside through the glass with magnets. They need glass of the right thickness and the maker's spacer, and the inside part must be lined up with the outside part.

What matters most isn't which pump you use, but how you use it: the angles, the modes, and how the flows meet (sections 4 to 7).

Instead of one small propeller, one common gyre pump has a motor between two long fans, one on each side. Water comes out as a wide, flat sheet. Most of the intake is on the bottom, so it can be mounted high without pulling air into the flow. With both fans set the same way (A-B), they push in one direction; set against each other (A-A), they push in opposition. Swapping a fan arm reverses its direction.

It mounts on the glass with a magnet; a typical mount suits glass up to 15 mm, with stronger magnets sold for thicker glass. The side glass is the most popular place, and it can be mounted horizontally or vertically. Mounted on the side glass, it sends a sheet of flow along the length of the tank, and the water travels around in a circular loop: along the top, down the far end, back along the bottom. This keeps detritus suspended for the filters and reduces dead spots. Two pumps, one on each side, give corals flow from more than one angle.

When cleaning a gyre pump, note exactly how the fans and shields were aligned before taking them out; refitting them differently changes the flow in the tank.

A wave box is a box in one corner of the tank, with its top just above the water and a propeller pump in its lower part. When the pump runs, it pushes water out of the box; when it stops, water flows back in. Timed to the tank's rhythm, it builds a standing wave. The tank and stand must be strongly built, with room above the water for the box.

Wave boxes are largely redundant: a single pulsing wave pump can do the same thing with less hassle (Michael Maddox, President of IMARCS).

A surge device is a container beside or above the tank. A pump fills it slowly. When the water rises past the high point of a siphon (Module 12), the siphon starts and empties the container fast, sending a surge into the tank. Fit a safety overflow drain at the top in case the outlet clogs. Every part follows the plumbing rules: leak tested, and plastic, titanium or 316 stainless only.

In a closed loop, a pump outside the tank pulls water from the display and pushes it straight back into the display. The loop is sealed from where it leaves the display to where it re-enters; it doesn't go through the sump. It adds flow without any pump or cord in the display.

The tank has to be drilled for the loop's holes, and the plumbing planned, before the tank is set up: it's a feature to commit to at the start (Module 36). The intakes and outlets have to be hidden in the rockwork. A screen over each intake keeps animals and debris out. Unions on both sides of the pump let it come out for cleaning, and a valve can set the flow. Every rule from Module 12 applies: leak testing, and the material rule.

A closed loop manifold splits the loop pump's flow into several outlets, for example two at each end of the tank. Each branch has its own valve, so the flow from each outlet can be set (Module 12).

Devices can switch or move the flow between outlets, making it change like a wavemaker. Hobbyists use rotating outlets that turn and sweep the flow around; motorized valves that switch the flow from one outlet to another and can be wired to a wave controller (these can be loud); and water-driven switchers that need no electricity but restrict flow and can clog and stop switching.

After changing any branch valve, check every outlet again, because they all share one pump. Clean intake screens and switching devices during routine work; a clogged screen or a stuck switcher cuts or freezes the flow.

Plan the rockwork around where the pumps will go. Leave space on all four sides of the rockwork so water can get around it, and never put rock directly in front of a pump. A starting point for placement by tank length is shown above, with pumps above the top of the rockwork.

A dead spot is an area the flow doesn't reach. Detritus builds up there, and corals there do poorly.

To fix a dead spot: re-aim or move a pump; add a small powerhead aimed at that area; change a mode so flows cross; move or frag a coral that has grown into the flow; or, if the rockwork itself blocks the flow, rebuild it. Make one change at a time, then watch where detritus settles and how the corals respond over the next days (Module 1). Changing the flow can stress corals, so avoid drastic changes all at once.

Detritus is fish waste, leftover food and broken-down material. Kept in suspension, it reaches the overflow and the filters (Module 15). Settled, it collects in the rock and sand and breaks down there (Module 8).

Read it: look at the tank before morning maintenance. Piles on the sand point to dead spots, so move or re-angle a pump; detritus sitting on the tops of rocks points to flow that's too laminar, so switch to random or pulse modes.

Ways to reduce it: standing waves, which keep detritus suspended through the whole tank (section 6); alternating pumps; gyre flow; a nutrient transport or tidal swell mode that sends a surge toward the overflow; aiming flow behind and under rockwork; and manual removal: removing collected debris every week is the most effective option, with no new equipment needed.

Anemones move around, and one that reaches a pump intake can be pulled into it. Small fish and fry are at risk too. An intake guard fits over the intake: its small holes keep animals out, and it spreads the suction over a larger area, farther from the blades. A rigid guard with many small holes clogs less than a foam guard, so flow drops less.

Check the intakes during routine work. A guard that clogs cuts the flow, so clean it with the pump. If an anemone is against an intake, switch that pump off, gently free the anemone, and fit a guard before the pump goes back on.

The parts: the motor turns a magnet in the rotor, which spins the impeller (or propeller) on a shaft, often ceramic. The housing holds it all, and an intake screen or cage keeps debris out. Never run a pump dry, and unplug it, including from any battery backup, before opening it.

Signs a pump needs cleaning: weaker or uneven flow, corals not extending as usual, humming, grinding or rattling, a pump that doesn't restart after feed mode, or a body that feels warm. The most common cause of pump noise is dirt on the wet side.

To clean it: unplug it and take it out. Take it apart following the maker's instructions. Rinse in warm RO water. Soak in white vinegar and water, half and half, or citric acid at 2 to 3 tablespoons per gallon (3.8 L). One to two hours is usually enough; for heavy buildup, up to 24 hours is safe. Never use strong acids; they can damage the parts. Brush the impeller, shaft, intake and magnet with a soft brush. Rinse in RO water, rebuild, and watch it start.

How often: follow the maker's interval, usually somewhere between every month and every 6 months, and every 4 to 8 weeks in heavily stocked tanks. Clean sooner when you see buildup.

Inspect the impeller every time you clean: look for cracks, chips, a loose magnet and worn or broken blades. Limescale buildup can crack an impeller arm; regular vinegar soaks prevent it. A damaged impeller is replaced, not repaired.

The shaft wears over time. On some pumps the ceramic shaft is the main wear part; replace it at about four years, even if the pump still runs. Rubber feet absorb vibration; a pump that rattles against the sump may need its feet checked or replaced.

Won't start: check the power first. Then unplug it and look for an obstruction or damage at the impeller, and clean it. For a DC pump, reset the driver following the maker's steps. If it still won't run or stays noisy after a full cleaning, replace the worn part from the maker's spare parts, or the pump. Keep spares for critical pumps (Module 36).

When the pumps stop, surface movement stops, and oxygen stops coming in. Waste stops moving to the filters, and the return pump stops too (Module 12). Battery backups are recommended for flow pumps so some movement continues during an outage, and many DC pumps work with one.

When the power comes back, check that every pump restarted: return, flow and skimmer. A dirty pump may not restart, and a pump that stalled may need cleaning before it will run again. Module 33 covers power cuts in full.