The flow in your marine tank

Taking the mayhem out of marine fish keeping

The Marine Tank Environment

Flow in your marine tank


The flow in your marine tank is very important for the health of your marine tank inhabitants. It serves a variety of purposes including:



Bringing food to invertebrates

Taking waste from invertebrates

Preventing the build up of detritus

Helps to reduce the build up of bacteria that can cause infection

Creates surface agitation


As you read around you will often see 'turnover' rates mentioned 10x, 20x etc.This is the number of times all of the water in your marine aquarium is pushed through the pumps or powerheads. I should mention here that a pump draws water in and pushes it through a piece of equipment such as a protien skimmer, a powerhead simply draws water through itself and pushes it out of the other end for circulation.


The amount of flow you need in your tank will vary depending on which species you intend to keep and it can be difficult to cater for everything in the one tank.

A FOWLR tank or an SPS tank will benefit from a high turnover of water (some suggest up to 40x) so if, for example you were keeping a tank with a water volume of 400 litres you would be looking at installing enough powerheads which combined add up to 16,000 litres per hour. All powerheads are rated for litres per hour so in this case you would look at something like four powerheads each capable of moving 4,000 litres per hour, or two for 6,000 per hour and two for 2,000 per hour. You are much better off with a few smaller powerheads to help create variations in flow rather than one large one which will probably just reult in you having to peel everything off one wall of the aquarium after its been blasted to oblivion from a single, massive flow of water.


A typical soft coral tank doesn't require as high a flow rate and many soft corals will not take too kindly to being blasted about from one side or the other. Aiming for around 20x per hour would probably suit them well. LPS corals will be happy with somewhere between the two. Of course depending on how you place the powerheads chances are there will be some areas of the tank with high flow and some with low, it's this variation that will allow you to keep flow loving corals in one part (SPS) and possibly some LPS or soft corals in another part.


If you are intending to keep seahorses and their suitable tank mates it is worth noting that they don't like much flow at all and youu would be aiming at no more than about 10x.




Filtering your marine tank is a subject worthy of it's own page (see the Filtration page) so here I'll just say that flow within your tank is one of the most important elements of effective filtration. It's also important to ensure there is no build up of detritus which could result in elevated levels of nitrate and phosphate as the waste products are broken down in these 'dead' spots far more slowly. These dead spots are also more likely to provide ideal conditions for undesirable bacteria that can lead to infections of both your fish and your corals.


Bringing food to, and taking waste from, invertebrates


SPS corals rely on a good flow around the tank to bring them the elements and food they need to thrive. If you decide to supplement their food with one of the many on the market you would just put it in an area of the tank with a high flow rate and let the water movement do the rest, this is obviously more effective if there is a high flow rate throughout the tank.


All corals produce waste and mucus and soft corals in particular like toadstools shed their outer skin. They rely on the water movement in the tank to take this waste away, the longer the waste products are hanging around the corals the more damage it can do to them.


Surface agitation


This really comes under the filtration category. Once the bacteria responsible for your filtration have done their thing there they produce byproducts of nitrogen gas. Braking the waters surface using the flow from the powerheads ensures this gas can escape and the much needed oxygen from the atmosphere can be drawn into the aquarium. Waste from your tank inhabitants will also increase the levels of carbon dioxide in the tank (which in turn will lower the pH), again ensurring the surface of the water is agitated will help to releae the carbon dioxide from the tank whist allowing oxygen to be taken in.


There are products on the market called wavemakers. These do exactly as the name suggests and turn pumps on and off for either timed or random periods. The irregularity of the water flow seems to benefit many marine organisms but I will mention here that they are not essential and if you are building a marine tank on a budget you will be fine with some of the cheaper end powerheads that combine to an adequate flow rate



Helpful tips


Flow in your marine tank


Try to use a few powerheads to make up adequate flow rather than one large one


Try to avoid too many dead spots where bad bacteria and detritus can build up

Flow in your Marine Tank - in a nutshell

The flow in your marine tank is one of the most important elements contributing to your filtration

Soft coral tanks can be run with a flow rate of around 20x per hour

SPS and FOWLR tanks should be run with a flow rate of around 40x per hour

A powerhead takes water in and pushes it out to create circulatioin. A pump usually draws water in and pushes it through a piece of equipment

Always make sure there is some sort of protecting guard on your powerheads, it's too easty for some marine organisms to get caught up in them


Make sure your powerheads are located in easy to reach places, you will have t ocarry out general maintenance and cleaning so trying to hide them behind a 40 kg wall of rock isn't necessarily a great idea

The surface of the water must be agitated enough to allow gaseous exchange which is an essential piece of your tanks filtering process



If you decide to keep something like an urchin check your power wires regularly as they can (and do) merrily munch their way through the protective outer coatings