This topic is mostly discussed on reef tank forums but it really applies to all aquariums. Just how much flow do you need? Seems simple, but it is not an easy question to answer. More important may be the question “What kind of flow do I need?”
There are actually many types of flow.
Flow isn’t just flow, it isn’t just the movement of water. Of the at least 13 types of flow there are two that are of particular interest in the aquarium hobby – laminar flow and turbulent flow. The below YouTube video is great for explaining but in case you don’t have time to watch I will give you the basics.
Laminar flow is smooth and consistent. The layers of flowing water do not collide with one another, they move in the same direction. There are obvious limitations to this. At some point the streamlines will impact an object and that will alter the flow pattern somewhat. In our aquariums this can be rocks, or the walls of the tank or even the fish themselves. But this disruption is tremendous and the streamlines will quickly find a pattern. Laminar flow is easy to accomplish and allows for some excellent circulation in a tank. This is the type of flow I have used for most of my tanks. Though not 100% accurate you can think of it as coming out from your powerhead, moving along until it hits the far wall of your tank, moving up, then heading back to the start, like a circle. You will notice a connection between “circle” and “circulation”. The root is the Latin root word “circ” for “ring”.
You may have already noticed that “laminar” and “laminate” share the same root as each other. That root tells us the thing is split into layers. It’s difficult to think of water as having layers, but it is important for us to get that. Water at the top of your tank can move slower than water at the bottom if you so choose. You can have a powerhead at the top moving slowly and one at the bottom moving quickly. Or simply one at the bottom and the water will naturally slow as entropy takes over. We will talk about why that matters after we discuss what turbulent flow is.
Turbulent flow is marked by the irregularity of the flow. It does tend to eventually form a pattern but in a tank we aren’t really going to identify it. Rather than being in layers the changes are chaotic. You won’t produce true chaos in your reef tank obviously but the idea is that the flow isn’t smooth and changes and powerhead flow “bashes” the streamlines into each other. The Latin root of turbulent goes back to words associated with a crowd. Basically a mob of people doing whatever they want. If we pay close enough attention we can see purpose and pattern but it is so complex doing so is not easy.
Are there others that we might care about for our tanks?
The short answer is “yes” but they tend to be similar to the above two, or unattainable in our aquarium environment in any meaningful way. Some powerhead or powerhead adapters can use other types. For example adapters that spin, or that compress the stream like your return lines do.
Aside from type of flow there is also the matter of velocity
In the hobby you will hear “high flow” and “low flow” which refer to the velocity of the flow. How much water passes a certain point during a given time. You will see your powerhead is rated this way 500gph or gallons per hour. That means that 500 gallons of water pass through the powerhead in an hour.
Why do we care about flow?
That is a great question. It is probably a question you come to if you have read any books or been on any forums because the subject is discussed often and sometimes angrily. You will quickly pick up on the fact that it is important but may not be sure why. The why isn’t easy, and that is some of the reason you see this topic discussed in a confusing manner and with some emotion. People don’t always agree because they don’t always have the same set up.
We care for two reasons really.
- Our livestock require a certain flow
- Flow helps keep our water in good condition
Number one there is what really causes all the issues. I have kept river fish, fish used to tidal forces, and I’ve also kept corals that require a certain flow to be happy. Some corals do well with high flow and others with low flow. SPS do well with high flow and tend to actually do pretty well with turbulent flow. This is because their structure promotes the depositing of detritus within the branches of the coral and the flow can help keep the coral free of debris. Just in your SPS can really cause health issues with the coral tissue. Corals like xenia tend to do well with lower laminar flow. They close up at night which helps dump off detritus and use their tentacles, which often pulse, to gather food. If you have a mixed tank, I tend to recommend higher, laminar flow. This means that the environment and livestock of your tank determines your flow needs.
Number two is a matter of getting the gunk out of your display tank and into your sump. It is also a matter of oxygenation and preventing dead spots which can lead to excessive algae or bacterial growth. One of the best ways I have found to get rid of cyanobacteria is to increase the flow. Food that settles to the bottom can be kicked up by the flow where fish can then consume it.
Can I mix up flow types?
In almost all cases the answer to this is “absolutely!”
There are lots of ways you can do this. The easiest, but probably most expensive way, is with a controlled powerhead like a Vortech MP-10 (or MP-40 etc.). These powerheads come with controllers which will periodically switch from laminar flow to turbulent flow and then back again. This mimics the ocean’s tidal regions well and what happens at high and low tide. Great if you have corals.
Fish also can sometimes use a rest and such powerheads can reduce speed at night to give fish a sleep time.
If you keep planted tanks you know that some plants don’t do well with high flow, or turbulent flow, so plants are another reason to choose one over the other. Fish that live in rivers are used to the water going one direction pretty much all of the time, either at high velocity or lower.
If you can’t afford an expensive powerhead there are cheaper models that do similar things and also controllers that are separate.
An innovative way to do this on the cheap is to buy smart plugs. I have tons of these. They hook up to your phone via app and also via Amazon Alexa. If I say, “Turn off fish tank two” the pump shuts down, I can then feed my fish. If you have several powerheads, just regular ones, you can set a schedule on the app which will turn pumps on and off at designated times. You can have laminar flow then turn on a pump or two facing a different direction to creating conflicting flows and thus turbulence. As a side note I also use these on my sump lights to turn them on and off. When they first started making smart plugs they were bulky but now are streamlined and you can even get a smart power strip where each outlet is controllable.
Flow is important but what kind of flow depends on what your livestock needs, including corals and plants. But regardless you need to think about flow, figure out what is best. When approaching this subject on forums I have found it is always best to start with a description of your tank, or if just starting what you hope your tank will be. If you post, “I am building a 30 gallon tank that will be filled with guppies what flow should I have?” you will find a better answer forthcoming. Being vague makes it hard and also keep in mind what I mention above about mixed tanks. That will make it much harder. When asking on forums about that list what you have or plan on having and ask specifically what those with mixed tanks have found successful.