It is said that although some corals survive without light, no corals can survive without flow. Water flow brings corals their food and washes away their waste. They also use currents to reproduce, releasing eggs and sperm into the water column which travel around before settling and finding a permanent home. So flow is one of the fundamentals of a healthy reef aquarium.
A reef aquarium receives flow in two ways, from the return pump and from flow pumps. Return pumps are what powers the filtration system, and is typically one pump sat in the filter section at the back or in the sump underneath.
Although some have adjustable flow patterns, most run at a constant speed and flow back into the tank via an outlet pipe or nozzle. The rating of the return pump will typically be stated on it, like 2000lph for example. But that will be its maximum output so if it’s underneath the tank it has to pump the water up out of the sump and maximum flow will be reduced in something called head loss.
Flow pumps are additional pumps that are placed in the main display to increase circulation. Also known as wave making pumps, flow pumps have wide outlets and high outputs, typically 5000lph or more, and are specifically designed to move lots of water and create broad currents for corals.
Many flow pumps have adjustable and even controllable flow, so not only can the flow rate be turned up or down via a controller, it can also be made to pulse, alternate with another of its kind on the opposite side of the tank or even create random flow patterns and waves, hence the name wavemakers. By having flow pumps and a return pump, the return pump can be set to the best speed for filtration while the wave-making pumps can do dedicated, separate flow for corals.
To find out how much flow you need, first, you need to know the volume of the aquarium, like 250 litres for example. If it’s being powered by a 2500lph return pump then the water in the tank is being turned over 10 times per hour. For soft corals and LPS corals aim for at least 20 times turn over, and for SPS corals, at least 50 times tank volume turnover per hour. So for a 250 litre reef tank, it needs a minimum flow of 5000lph and for SPS corals it would need 12500lph.
The amount of decor you have in the tank, and how its placed, will negatively affect flow. In a bare tank, flow circulates around the tank, only bumping into the tank sides and the weir, but when you place rocks in the tank they act as flow barriers. Place corals on top of the rocks and they also block the flow, acting like windbreaks, so many reefkeepers find themselves having to purchase more flow pumps as their tanks become more and more heavily populated with corals.
To lessen the need to buy twice, purchase a controllable flow pump with a maximum flow which is bigger than you need. This way you can start the pump at say 40% output and as the tank becomes more cluttered, flow can be increased.
Can you get tiny flow pumps for nano tanks?
Yes, flow is just as important to corals in nano reefs, and there is a variety of tiny flow pumps to choose from. AC pumps are cheaper, and reliable, but only offer one flow pattern. DC pumps come with controllers to enable all the flow settings of larger, more sophisticated reefs.
Can you get a pump with App control?
Yes, some high-end, controllable flow pumps also come with App control from mobile devices. It’s a nice feature to have and depending on the brand, lighting may also be controllable via the same app.