dosing phytoplankton in a marine aquarium

Dosing phytoplankton in a marine aquarium

The subject of dosing phytoplankton in a marine aquarium is a popular debate in online forums. Generally, there are advocates in favor of dosing phytoplankton and those who believe the potential risks outweigh the benefits. This article will cover several topics related to dosing phytoplankton in a marine aquarium, including:

What are phytoplankton?

Phytoplankton (also called microalgae) are small photosynthetic organisms (similar to single-celled plants) that make up the bottom of the food chain. Drifting along in the ocean, phytoplankton turn light energy from the sun and carbon dioxide into the life-sustaining sugar called glucose. In fact, the name phytoplankton is based on Greek words for “plant drifters” (according to Wikipedia) to further characterize their floating plant-like life. Like land plants, phytoplankton convert light into sugar by a process called photosynthesis.

Phytoplankton are considered primary producers in the food web, which means they make their own food and that many other organisms depend on them as their food.

Check out this primer on phytoplankton if you want to learn more.

Arguments for and against dosing phytoplankton in a marine aquarium

For Dosing Phytoplankton

On a natural coral reef, phytoplankton are an abundant food source for many clams (and other bivalves), soft corals, sponges, and zooplankton-like copepods. In the home marine aquarium, however, phytoplankton are generally not present. Those aquarists in favor of dosing phytoplankton, therefore, are attempting to provide this natural food source to the invertebrates in their tanks.

The argument for dosing phytoplankton is generally based on two principles:

  • Phytoplankton are a natural food for many filter-feeding invertebrates
  • Dosing phytoplankton provides a nutritional boost for the clams, filter-feeding corals, and other beneficial invertebrates like copepods in their marine aquarium

Against Dosing Phytoplankton

Of course, there are also aquarists who are opposed to dosing phytoplankton in a marine aquarium.

The argument against dosing phytoplankton is usually based on two principles:

  • Many of the coral and invertebrate species in a marine aquarium are not thought to directly feed on phytoplankton, which could make dosing phytoplankton a waste of time and money.
  • Dosing phytoplankton in a marine aquarium also adds phosphates, nitrates, silicates and can actually cause water parameters to deteriorate in an otherwise healthy marine aquarium.

Do corals need phytoplankton in their diets?

Whether or not corals need phytoplankton in their diet has been a topic of debate for some time. Many soft coral species are thought to depend on phytoplankton for a significant percentage of their nutrition. An article from suggests that many of the soft coral species that feed on phytoplankton possess pinnula, tiny feather-like extensions on their polyps, like the polyps on Xenia.

The jury appears to still be out on stony corals, by comparison.

According to an article by Eric Borneman on, multiple research studies on stony corals have produced conflicting results.

One study reported that Acropora, Montipora, Porites, and other stony corals ingest (or at least clear) phytoplankton from the water—while another suggested that only Astrangia and Porites ingest phytoplankton (Source:

While not every species of coral will feed on phytoplankton, it does appear to be a beneficial food source for many species. Phytoplankton are also a major food source for tiny invertebrates like copepods—so dosing phytoplankton may provide a secondary benefit to predatory corals and fish in your aquarium. The decision about whether or not to dose phytoplankton in your aquarium, therefore, is going to be a personal decision about the relative benefits compared with the cost. If you choose to dose phytoplankton in your marine aquarium, there are two common ways of doing so.

Two Common Approaches to Dosing Phytoplankton in a Marine Aquarium

Direct Dosing

Direct dosing phytoplankton is the technique used when you aim a tiny burst of phyto directly towards the extended polyps of the targeted coral or clam. To dose phytoplankton directly, you need an eyedropper, turkey baster, or pipette to extract a small amount of phyto from the bottle. The next steps are fairly straightforward:

  • Use your eye-dropper or pipette to suck up some of the phyto
  • Slowly move the tip of the dropper next to the target coral or clam
  • Gently squeeze the bulb at the top of the eyedropper to carefully release the phytoplankton contents inside the dropper
  • Repeat this step for every coral you intend to feed

The advantage of direct dosing phytoplankton is that since you are target-feeding your coral, you only use a small amount of phyto, which keeps costs and the risk of deteriorating the water quality low. The disadvantage of direct dosing is that it is time and labor-intensive (relative to indirect dosing). The benefits to copepods and the other tiny invertebrates in your live rock and sand are minimal.

phytoplankton culture bottles


Indirect Dosing

Indirect dosing phytoplankton, by comparison, is a technique used when you add the phyto to the water column without targeting any specific coral or invertebrate. While the overall volume of phytoplankton dosed may still be small (when compared with the tank volume), the dose administered for the indirect method should be much larger than when direct dosing—because the concentration of phytoplankton will be diluted by the large volume of your aquarium.

The method you choose for dosing phytoplankton in your marine aquarium is going to be a personal decision you have to make—based on your own style and beliefs about the benefits of the phytoplankton to the inhabitants in your tank, as well as the level of involvement you want to have in the meticulous feeding of these organisms. If you are unsure which method will be right for you, consider alternating between the indirect and direct methods.

Using a protein skimmer when dosing phytoplankton

Another important thing to remember is that your protein skimmer if left on while you dose phytoplankton for your corals, will skim the phytoplankton out of your aquarium. So it is best to turn off your protein skimmer before you dose phytoplankton into your marine aquarium. Just remember to turn your skimmer back on when you are finished, or you’ll have bigger problems than you had bargained for.

How to dose phytoplankton

Manual dosing phytoplankton

If you do not grow your own phytoplankton or purchase commercial products infrequently, then manual dosing will likely be your preferred means of adding the phyto to your marine aquarium. If you plan to dose indirectly, the task could be as simple as measuring out a few capfuls of the stuff and pouring it indiscriminately into your tank. If, on the other hand, you want to feed specific corals or clams, then you will need an eye-dropper, turkey baster, pipette, or another tool to help you disperse the phyto right where you want it.

Advantages of manual dosing

The advantages of manual dosing are:

  • As long as you have hands, you can dose phytoplankton
  • No added cost and technical complexity from dosing equipment
  • You also never have to worry about pump failure.

Disadvantages of manual dosing

There are a few disadvantages of manual dosing:

  • Unless you schedule a routine, you may find it difficult to dose phytoplankton with any consistency.
  • The task of manual dosing may become a chore.
  • Opportunity and risk for human error are very high–you might…
    • Leave the phytoplankton out overnight, causing it to spoil
    • Spill the bottle
    • Accidentally overdose your tank

There is no guarantee you will do any of that–but the more often you manually dose, the more opportunities you have for something to go wrong.

Automated dosing phytoplankton

If you are a little more tech-savvy, have a slightly bigger budget or value consistency for your saltwater aquarium. Automated dosing of your phytoplankton may be the best option for you. The basic principle here is to set it and forget it. You set up a reservoir, which is just a fancy word for a container with the phytoplankton you want to dose, connect some tubing and a pump. You program the pump to slowly, steadily drip phytoplankton into your aquarium until the reservoir is emptied. The only work involved is setting up the drip, refilling the reservoir, and performing maintenance, if needed, on the pump.

Advantages of automated dosing phytoplankton

The advantages of automated dosing phytoplankton are:

  • Maintaining consistent levels of phytoplankton in your system is likely better for your aquarium than the ups and downs associated with manual dosing
  • Once you setup your system, the work involved is minimal
  • You eliminate a significant opportunity for human error by automating your system

Disadvantages automated dosing phytoplankton

The disadvantages of automated dosing phytoplankton are:

  • Setting up an automated system adds complexity and cost to your aquarium
  • Pumps and hoses may fail eventually, causing a mess when it does happen

Dosing equipment

For manual dosing

A bottle of DT`s Phytoplankton, or equivalent

An eyedropper, turkey baster, or Julian’s Thing.

For automated dosing

  • Reef dosing pump–you have the option to select a single dosing pump or a multi-pump unit. These can range from about $100 to $500, depending on the number of pumps and features.
  • Tubing
  • Bottle or reservoir

As you can see from the equipment list–neither option is over-the-top complicated. I have seen mentions of connecting single-unit pumps with electric on/off timers to program an analog pump or put in a fail-safe–but that is beyond the scope of what’s intended.

How to store phytoplankton

Phytoplankton cultures you intend to feed to your marine aquarium are generally best kept in the refrigerator.  While the cultures themselves were probably grown at an aquaculture facility at room temperature, the fact is that a thriving phytoplankton culture typically grows at a less concentrated level than the culture in the bottle you bought at your local fish store. If that bottle was to warm up to room temperature, the phyto would most likely die and foul up the water in the bottle. If your phytoplankton culture smells spoiled, it probably is. Dispose of it and start over with a fresh bottle. For what it’s worth, this is the single biggest hassle I have encountered with dosing phytoplankton. The chore of placing the bottle back in the refrigerator when finished is easy to forget and regret.

Another consideration when storing phytoplankton in your refrigerator is that the phyto will settle to the bottom of the bottle. If the tiny cells of phyto that are stuck at the bottom of the bottle remain stuck there too long, they will die. When they die, they will also foul the water inside the bottle and cause the whole bottle’s contents to spoil. Make sure you shake up your phytoplankton culture every few days to keep the phyto suspended in the water column rather than layered on the bottom of the bottle.

DT’s Phytoplankton brand is one of the more commonly available brands of phyto at your local fish store. While they clearly have an incentive to promote phytoplankton use in marine aquariums, they do have some interesting information on their website. One section they call Phytoplankton feeding trials, which shows some interesting data supporting a boost in growth seen when phytoplankton were added.

Growing a phytoplankton culture at home


Perhaps the best place to get fresh phytoplankton to dose in your marine aquarium is to grow your own phytoplankton at home.

growing phytoplankton culture at home

It’s easy to do, inexpensive, and only requires equipment and salt mix that you would probably have around your house anyway (assuming you already have an established marine aquarium). A straightforward setup can get you started. Phytoplankton require light, fertilizer, carbon dioxide, and oxygen to grow. Once you start growing the phytoplankton at home, you can harvest each bottle about once a week.

In addition to being used as a nutritional supplement for the corals, clams, and other invertebrates in your marine aquarium, you can use your phytoplankton culture as the base for other home cultures including rotifers and brine shrimp. Growing phytoplankton at home is also typically a necessary step in breeding saltwater fish since the phytoplankton are a required food source for the rotifers and brine shrimp you want to feed the newly hatched saltwater fish larvae.

Written by Albert B. Ulrich III.

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