Basics of Flow

Discussion in 'Reef Aquarium Articles and How To's' started by mikejrice, Dec 17, 2009.

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  1. mikejrice

    mikejrice 3reef Affiliate

    May 24, 2009
    Basics of Flow:

    One of the most under-rated things to consider when setting up a marine aquarium is flow. Water movement is essential to maintaining a healthy aquarium including: fish only (FO), fish only with live rock (FOWLR) and reef tanks.

    The biggest purpose of good flow is controlling detritus. With enough flow, detritus is not allowed to settle to rest on the substrate of you aquarium. If it is allowed to settle it will decompose and create nitrates. By keeping all the detritus in your tank suspended in the water column, you leave it no place to go but into the filtration. The filtration cartridges can then be cleaned ridding your system of the nitrate causing detritus.

    In FOWLR and reef systems having ample flow becomes even more paramount. Due to the live rock being the main filtration in these types of tanks flow to get water to the bacteria is very important. An even higher flow rate than is needed to keep detritus moving is recommended when using live rock as filtration.

    How much flow?

    Flow is rated in total tank turnovers per hour. This is the amount of times the total tank volume is run through your pumps/power heads per hour. To calculate turnover rate, use this basic equation:

    Total flow of power heads/pumps ÷ Total tank volume


    If a 10 gallon tank had one power head pushing 200gph, then the formula would be:

    200 ÷ 10 = 20x turnover rate per hour

    FO tank flow:
    FO tanks generally need the lowest flow. From 15x turnover to 20x turnover should be enough flow to keep detritus suspended.

    FOWLR flow:
    FOWLR systems will need more flow to get water to the filtering bacteria. 20x turnover is a recommended minimum.

    Reef flow:
    Reef tank flow will start out at the same FOWLR guideline of 20x turnover, but depending on coral type, could be all the way up to 100x turnover or more.

    Coral and Flow:

    There are many different types of coral which demand different types and different intensities of flow. Many of the beginner corals will do well in the minimum 20x turnover rate and will happily convert to much higher flows to accommodate new additions later.

    Low flow (20x-30x):
    Several types of corals do very well in low flow conditions such as: zoanthids, palythoas, some leathers, most large polyp stony (LPS) and many others.

    Medium flow (30x-40x):
    Most of the corals from the low flow group will also do well in medium flow. Medium flow also adds a few very fast growers such as xenia and more species of leather coral.

    High flow (40x-100x+):
    Some of the low and medium flow corals will do ok if placed out of direct flow up past the 50x turnover mark. Corals from the medium flow group such as: xenia and leathers will still thrive in high flow. Fleshy LPS will have to be placed in a relatively dead spot to survive in a high flow aquarium. 50x+ turnover introduces a whole new group of corals known as small polyp stony (SPS). SPS coral needs very high flow to keep its polyps clear of waste.

    Type and Direction of Flow:

    FO and FOWLR tanks will do well in most types of flow as long as there is enough. Reef systems however demand more chaotic flow to keep coral healthy and growing. To create a more chaotic flow for your corals you can position power heads so that flow converges in one spot, set pumps on timers or use a wave maker.

    Laminar Flow:
    Laminar flow is unidirectional. You will usually end up with this type of flow when you have only one source of flow. Very few corals such as sea fans thrive in this type of current. You can get away with laminar flow in a FO or FOWLR, but it may lead to detritus settling in dead spots.

    Alternating Flow:
    Alternating flow is most comparable to a wave motion. The water pushes forward then back in a rhythmic motion. This is the type of flow you will get from a wave maker or some timers. This is a good flow for all around coral health. You can push a lot of flow in an alternating pattern without injuring LPS and other low flow corals. This flexibility with pushing a lot of flow while it also being healthy for low flow corals makes this the best choice for a mixed reef.

    Random Chaotic:
    Random chaotic flow is usually the best choice. It is easily achieved with the use of several power heads pointed to converge in the tank. This type of flow is contently changing and thus does not beat the coral from one side all the time. Because corals are more commonly being hit with full flow from at least one side at a time, the usually restrictions on what types of corals will thrive in higher turnover rates will apply.
    Corailline, LakeShow, Dingo and 6 others like this.
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  3. NU-2reef

    NU-2reef Montipora Digitata

    Jan 27, 2008
    vancouver, canada
    great article mike. k+

  4. pafc64

    pafc64 Astrea Snail

    Jun 24, 2009
    South Australia
    Great information there Mike
  5. temp5374

    temp5374 Plankton

    Jan 2, 2010
    related flow question

    Hello, thanks for the flow information. I have been wondering for a while now how I can get a 20x flow in my 1000 litre marine tank. Does the 20x flow refer to all pumps in the system (as in powerheads as well as return flow from the sump) or does it mean that the filter (sump) should have the water pass through it 20x?

    If it does only refer to both the powerheads and the return pump from the sump, then how many times would you recommend that water flows through the sump each hour? I keep the matrix, skimmer, chiller and heaters in the sump. I also around 50 kg of live rock in the display tank.

    I realise that different corals etc need different flow rates, however I am not 100% which corals I will be getting at this point, other than I will not be getting hard corals because I do not have metal halides.

    So I guess the main question is how many litres an hour should my sump's return pump process and how many litres an hour should my powerheads process.

    Thanks so much for any help, I never feel like I can trust people at LFS because they give an answer even if they don't know what they are talking about.
  6. lunatik_69

    lunatik_69 Giant Squid

    Jul 10, 2007
    Miami, FL
    Great article Mike. K+

  7. mikejrice

    mikejrice 3reef Affiliate

    May 24, 2009
    It does refer to all pumps in the system.

    The main factor when thinking about this is the GPH of your skimmer pump. You don't want more flow through your sump than your skimmer can process.

    A good number to shoot for with your sump is about 10x per hour of the DT volume. The rest of the flow in your tank should be given via power heads. I would shoot for between 20 and 25 to start. It sounds like you are going to stick with softies and LPS so that should do pretty well for you. If you find that dead spots are developing you may want to add a bit more flow.
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  9. steve wright

    steve wright Super Moderator

    Feb 17, 2009
    shenzhen Guangdong PRC
    great article Mike

  10. adicus

    adicus Aiptasia Anemone

    Sep 28, 2008
    Mike - Great information! Follow up question. Most of the wavemakers/controllers have a "night" cycle where they slow down for a period of time. My MP40 night cycle is for ten hours. Should we be using the night cycles? From your information it sounds like we should not.

  11. mikejrice

    mikejrice 3reef Affiliate

    May 24, 2009
    I wouldn't use the night mode. Night time is when most corals extend their tentacles to feed actively on things in the water. They grow to suite the flow in your tank, so if you take some of it away you make it harder for them to catch food. I'm a fan of the thought that flow should be a bit different at night, but not less. If you have the time to do it each night and morning, I think switching your pump into lagoon mode at night would be a nice change for your corals. Some people even think the tidal changes at night may have something to do with coral spawning, so changing flow a bit at night could help reproduce that as well.
    1 person likes this.
  12. bje

    bje Long-fin Bannerfish

    Apr 12, 2009
    lmao the forum wont let me give you more +K... apparently i have to spread the love it says.

    i am a bit confused on the formula for figuring out turnover...

    would it be all of these items ? (dt-powerheads GPH + return pump GPH) + (dt volume + sump volume + fuge volume) = turnover

    so in my case: (900+300) + (75+8+6) = 1289

    in your example you had 200+10=210=20x turnover so whats 1289? 128x? that doesnt seem right to me. im having a hard time figuring this out for some reason. oh also we're not taking into account actual water volume in the DT. we need to subtract the rock displacement. cant really figure that out without emptying the tank and refilling it with a meter right? so we guesstimate, but either way i dont see how im getting 128x turnover. or is it 12x?