The Many Means of Nutrient Export in Marine Aquariums

aquarium-nutrient-exportDissolved nutrients. Sounds like a good thing, right? After all, every organism needs nutrients in one form or another in order to grow and stay in good health. So why are marine aquarium hobbyists—particularly reefkeepers—seemingly so fixated on keeping the level of dissolved nutrients in their systems as low as possible?

To understand this fixation, you have to keep in mind that the waters surrounding coral reefs are naturally nutrient-poor environments. Unless a reef is subject to agricultural runoff, sewage discharge, etc., the levels of dissolved nutrients around it never approximate what can accumulate in the closed system of a marine aquarium.

Elevated dissolved-nutrient levels lead to problems with nuisance algae and declining water quality, which is stressful or even deadly to marine livestock. That’s why hobbyists must implement different measures to export dissolved nutrients from their systems.

Here’s a sampling of basic nutrient-export techniques:

The routine water change

I’ve listed the water change first because it’s the most straightforward technique and provides many additional benefits beyond exporting dissolved nutrients. You should be changing a minimum of 10 percent every week or 20 percent biweekly (more if testing shows that nitrate, and/or phosphate is exceeding the acceptable level), siphoning out as much accumulated particulate waste as possible in the process.

Note: Ensure the source water you use to mix up your salt water and to compensate for evaporation doesn’t contain nitrate or phosphate. We recommend purifying tap water (e.g., via reverse osmosis/deionization) before using it in a marine aquarium system.

Protein skimming

A good protein skimmer comes in a close second to the water change when it comes to nutrient export. When you see the brown, smelly gunk that accumulates in the skimmer’s collection cup, you’ll understand why. The great thing about protein skimming is that it removes dissolved organic compounds from the water before they even have a chance to decompose and burden the biological filter.


Macroalgae, such as Chaetomorpha, grown in a sump or refugium under dedicated lights will take up dissolved nutrients from the water. Then, every time you harvest some of the algae, you’re effectively exporting the nutrients contained within the harvested tissues. Harnessing the nutrient-uptake potential of macroalgae is a great way to outcompete problem algal forms, such as hair algae, bubble algae, and BGA (aka, blue-green algae or cyanobacteria).

Rinsing or replacing mechanical filter media

Mechanical filtration, in and of itself, does not export dissolved nutrients. It merely sequesters particulate matter in some form of porous medium, which remains in the flow of water. If left in the filter medium for too long, the particulates begin to break down and actually cause the system’s dissolved-nutrient level to rise. However, regularly rinsing/back-flushing the filter medium and replacing it as needed will eliminate the particulate waste before it has a chance to decompose.

Live rock and DSBs

The use of high-quality, porous live rock and/or a deep bed (4 inches plus) of fine sand—either in your display tank or in a sump/refugium—can actually help export nitrate from your system. Deep within the structure of the live rock and in the lower reaches of the sand bed, colonies of anaerobic denitrifying bacteria thrive. These bacteria convert nitrate to free nitrogen gas, which escapes harmlessly from the water into the air.

What am I forgetting?
Do you use a nutrient-export method that I haven’t mentioned here? If so, please describe it for your fellow salties in the comment section below

Photo credit: Allie Caulfield


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. David Bowers says

    spot on: I would add a related couple of tid bits here. First make sure the skimmer is properly rated. For example: Most 90 gallon rated skimmers are no where near powerful enough for a 90 gal tank with several fish and a load of inverts.
    Second: Establish proper water flow within the system to avoid the accumulation of poo and uneaten foods etc. A great skimmer and religious water changes are wonderful tools. However, if the rock pile is getting gummed up with gunk from a bad pile design or improper flow then the keeper will still have nutrient issues.

    I think that was the toughest lesson I had to learn as I became a reef keeper. All aspects of the system have to work and work together.

    Lastly, use good quality foods, rinse them with care, and don’t over feed. Some foods are already nutrient waste lands before they start breaking down in your system. Don’t skimp on the chow

    your pal

    • Jeff Kurtz says

      Great insights, David! Thanks for sharing! Speaking of chow, isn’t it about time for another Fish Geek Dinner?

      • David Bowers says

        I am game! wings or a steak please 🙂
        You guys want to come to the lab or meet your way

        • Jeff Kurtz says

          Hmm, might be nice to head out your way to see what’s happening in the lab. Let me check with “Caribbean Chris” to see what he thinks and we’ll shoot you an email. Talk to you soon!

  2. I never understand why so many reef keepers try to alter the water chemistry so much with chemicals, rather than doing more regular water changes. Am i missing something here. I change my water every 2 to 3 weeks however the tank is very lightly stocked with fish and quite a few corals. Should i still be doing a water change weekly?

    • That is a real good question, the annoying answer is “it depends”
      I change 10 % a week but I need to. I have a heavy load with a lot of fish and corals.

      So how to answer your question: first we need to know more about your specific system. Are you having trouble with algae? Corals not growing as they should?
      Positive nutrients tests?
      If the answer is yes, then the answer MAY be that you need changes more often.

      The trouble/fun part of reef keeping is that systems are to some degree unique. If what you are doing is working then keep doing it. If not, then you should consider a change.

    • Jeff Kurtz says

      I completely agree with Dave here, Nick. With respect to your water-change regimen, if it ain’t broke, don’t fix it. Just be sure to keep an eye on your nutrient levels and factors like algae growth. You can always step up your water-change frequency or volume if you detect a worrisome trend.

  3. I have only a few fish and quite a few corals. Plus protein skimmer, refugium etc. presumably i am ok with a water change every few week, due to the lightly stocked tank. I also use a cannister filter when cleaning the tank and leave this on for just an hour. This is my extra method of ridding the tank of excess nutrients. I only leave it on for one hour as opposed to constantly so that it doesn’t become a nitrate factory.

    • I would think so too. I realized that I didn’t address your real question/comment. I think the reason for many guys altering their systems is the foolish belief that they can extend the duration between changes. Or that there is a magic potion that will make their corals grow faster. I don’t buy into this.

      Sometimes there are good reasons to add certain chemicals not fixed by water changes or dosing Ca and alk. Magnesium comes to mind.

      Overall though, I agree that it doesn’t make sense to pour in some elixir in hopes it will replace sound husbandry.

  4. Paul Baldassano says

    There is a big thing you forgot. Water changes, skimming, macro algae is great, but for long term tank survival, I don’t mean 5 or 10 years as that is not even the lifespan of a hermit crab, I am talking about 20, or 30 years. I am referring to the rock. Live rock is not a stagnant piece of expensive material that you throw in there and forget. It is a living, respiring organism and it needs maintenance. The pores in the rock are filled with bacteria but those pores clog, and clog fast with dead bacteria and pistachio shells from the nuts bacteria like to eat. We need to clean out those pores which is something I do at least twice a year using a diatom filter but any canister filter with a strong flow will work. I put the outflow against the holes in the rocks and you would be amazed what comes out of there. I can barely see through my tank when I do that. That is one of the secrets to long term success. Keeping pistachio’s out of the tank is another thing.

  5. A big thank you to every one. Years of experience shared is the most valuable thing one can procure aside from all the necessary tools in the world. We don’t usually realise this as it comes free. For those with these hard earned experience, the value is immense.

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