How to grow phytoplankton!

How to grow phytoplankton!

What do I need to get started?

1. Culture vessel

There are many choices here, depending on the size of your culture, personal preference, and availability. I don't personally think one culture vessel is better than any other, but am a fan of our 5 gallon carboys.

The most relevant factors to help you choose are size and ease of cleaning. For size, you should consider how much you are going to harvest, and how much you are going to be feeding. For most aquariums, Top Shelf Aquatics recommends a dose of 3 mL per gallon, or up to 5 mL per gallon if you have non-photosynthetic organisms to feed (gorgonians, electric flame scallops, other NPS corals).

Given you will be harvesting 3/4 of your culture on a weekly basis, you can size up accordingly. For example, suppose you have a 100 gallon tank. You need enough phyto to feed 300-500mL daily, so that would be 2100 mL to 3500 mL each week. Therefore, your culture vessel should be between 2.8L and 4.66L. Since most 100 gallon systems are not full of NPS corals, around a gallon (3.78541L) is plenty. As a good rule of thumb, we recommend sizing your culture vessels so that you have 1 gallon of phyto for every 100 gallons of water, giving you a good amount of extra to work with.

The next choice is the specific type of culture vessel you want to use. We have successfully cultured in 250 mL Erlenmeyer flasks, 1L flat bottom flasks, 1 gallon growlers, and 5 gallon carboys. The shape is not very important, some differences in aeration patterns and ease of cleaning should be considered. We love the 1 gallon glass growler for systems up to 135 gallons. Glass gives you other options for sterilization (including boiling) that you do not have with plastic, but is sometimes more expensive and is definitely heavier. For large systems, the 5 gallon carboy is great because it is very easy to clean (you can get your whole arm in there to scrub), and it even has a spigot at the bottom you can use to easily harvest your phyto.

2. An inoculant for your choice of phytoplankton

Next is to get your hands on a nice, clean inoculant for the species you wish to culture. There are many places to choose from here (including ourselves), so we recommend you do your own research and grab an inoculant that fits your needs. You can start with a petri dish (highly recommended, as this does help you get an isolated strain), 1 mL starter inoculant (you will gradually scale this one up with test tubes, flasks, and then place into your final vessel), or 8+ oz cultures you can purchase locally or online.

Which phyto should you grow? There are a ton to choose from! The easiest species to maintain are species like nannochloropsis occulata and tetraselmis chui. They are very resilient, and these cultures are typically not prone to pests. More difficult species include rhodomonas salina and T-isochrysis lutea. These are both extremely beneficial phytoplankton, and are full of nutrients, lipid proteins, and pigment enhancing proteins (they are used for this reason in many oyster / clam farming applications). One can also culture diatoms like chaetoceros with the same setups, but adding silicates to the media.

As for the size of your inoculant, I recommend 250 mL (or about 8 oz) per gallon.

3. Lighting

We use very simple lighting on a timer. We usually do 16 hours on, and 8 hours off. We typically just use an LED strip light that we wrap around the outside of the vessel and secure with tape. This is a rather crude looking setup, but it certainly works! It is also very power efficient, making this a no-brainer for us (and others who might not mind the resultant eye-sore). Aesthetics aside, the main downside with these is that you need to make sure you get the waterproof kind (or they will not last very long), and you need to remove them / replace them when cleaning the vessel.


Other solutions are flat LED lights that sit under the culture, and shine up through a clear bottom, or large LED lights / fluorescent bulbs that sit along the back of a culture, and shine through the side (this is perhaps the most common choice).

In general, the amount of light needed is not anything super powerful.

4. Aeration

The best thing to use is a rigid airline tube, and an air pump. You should not have violently thrashing water in your culture vessel, just a nice, even bubbling. Should be relaxing to look at. Do not use airstones attached to the end of your airline tubing, as this will clog quickly and is generally not going to move the water as much as having a bare airline tube in the culture.

The point of aeration is definitely gas exchange, but also to keep the culture suspended. Otherwise, live phyto can get trapped under a clump of dead phyto, which is not ideal for growth. If you notice a lot of precipitating phyto, you can stir your culture to resuspend them.

5. Microscope (optional, but important)

I don't know about you, but my eyes don't let me zoom in to see cells that are between 4 μm and 12 μm. If you can, please teach me.

The best way to diagnose any problems with your culture is to take a look! They are also fascinating organisms to watch, and have their own behaviors. Most phytoplankton are motile, and behave very differently. For example, here is a video of some Rhodomonas salina.

Very smooth, gentle swimmers... And, by contrast, Tetraselmis chui:

Then you have non-motile species, such as Nannochloropsis occulata. They are very small (about 1/3 the size of tetraselmis chui), and do not move very much at all.

How do I maintain my culture?

The simplest technique is to maintain a strict cleanliness regimen at all times, and to consistently harvest the algae on a weekly basis. For those cool kids with multiple cultures to care for, we recommend doing them on different days of the week.

Phytoplankton can last for up to a month in the refrigerator so long as you turn the culture over every day (or at least when you remember to) in order to keep the cells suspended. I have even frozen a backup culture by accident previously, and the culture was still alive when it unfroze (not a technique I recommend).

In general, I'm sure you can find local hobbyists who will be more than happy to take some off your hands should you find yourself in excess.

1. Start with an empty culture vessel

The first step is to clean out the vessel you are going to use. If your culture is full of phyto, we are going to harvest 3/4 of culture for use. Otherwise, you can pause for a moment and think about how nice it will be when your culture is full of phyto a week later. Of this 3/4, I highly recommend you keep a backup, should your culture crash (crashes typically happen very quickly, and with very little warning...)

Take that 1/4 that remains and move it to a sterile container while we clean out the rest. Ideally, you avoid the junk at the bottom, or the junk that builds up on the spigot (if you are using a carboy) when you collect this sample.

To clean your container, give it a really good scrub with some warm, soapy water and a brush. Try to make everything look brand new and shiny, and remove as much of the soap residue as possible. The ideal soap is a detergent such as Alconox™, but regular dish soap works just fine in our experience. If using Alconox™, do be careful to avoid contact with your skin, and only use a very small amount. This is a powder soap, and is irritating to the lungs if accidentally inhaled, and generally should not make contact with your skin due to the alkaline nature of the soap. It is, however, a great choice for glassware due to the lack of residue or smell after use.

2. Mix clean saltwater for use

We generally mix the saltwater in our freshly cleaned culture vessel, and mix the salt up to 35 ppt (though we just recommend whatever you use typically). Specific phytoplankton can tolerate different ranges of salinity (e.g. Nannochloropsis can tolerate freshwater), so we recommend doing some research if you are going to push this to an extreme. Generally, though, we focus on saltwater for our aquariums, and choose 35 ppt for this reason.

Your specific choice of salt is not very important. Most fish-only salts are fine, and there is no reason to break the bank on super high quality reef salts for growing these. We like aquaforest sea salt, as it is comparatively cheap and mixes clear in a short amount of time, but many, many other salt mixes will get the job done.

Note: you should only fill the container 3/4 of the way, as you will be adding back 1/4 of the culture volume that you collected previously.

3. Sterilize the setup

There are primarily two options for sterilizing at home. If you have a hot plate and a magnetic stirrer with a glass container, you can just boil the water inside the culture vessel for a few moments. If you do so, mix slightly lower on the salinity, as you will evaporate water while boiling. If you go this route, you should wipe down all your airline tubing with an alcohol (we typically use denatured alcohol, but any >90% isopropyl or ethanol is fine). Do not try to boil water in a plastic container.

The other option is to assemble the entire setup, get everything running, and add bleach. We then neutralize the bleach with sodium thiosulfate. You can either use a salt form (such as sodium thiosulfate pentahydrate), or you can purchase our 0.25M pre-mixed solution.

We like to add bleach so that the water is 100ppm NaClO (sodium hypochlorite). To figure out how much you need to add, first find the concentration of your bleach. This is usually a percentage that is visible on the front of the bottle.


In this case, we have 7.55% bleach. Most household bleach products are around 7.5%, so just check the label to be sure. Avoid any bleaches that are scented, or have other "color preserving" features, we just want plain, household bleach.

To calculate the amount you need, we have a convenient calculator below.

Bleach Calculator


Once you have the amount of bleach you need, just add the bleach to the vessel, turn on all the air pumps, drop in your airline, and let it run for an hour or two.

Next, neutralize the bleach with the sodium thiosulfate. Using the liquid solution should do so almost instantly, and you should lose the bleach scent very quickly. If you use the salt version, dissolve the crystals completely. Wait 15 minutes just to make sure all the bleach is neutralized. When it comes to sodium thiosulfate, It is better to overdose the than to under-dose. We typically dose a bit more than necessary out of an abundance of caution.

4. Return the Phyto to the culture vessel

Since everything is all cleaned up and ready to go, add the 1/4 you set aside into a sterile container earlier back into the culture, or (seed the culture with your inoculant for the first time). Depending on the density of the culture, the amount of inoculant used, the culture may appear quite thin. For example, this is what 1 L of Rhodomonas salina inoculant looks like in 5 gallons of sterile saltwater.


5. Add f/2 fertilizer

The next step is to create f/2 media by adding f/2 fertilizer. If you want to do this yourself, you can find a great PDF outlining the process here. To be frank, though, it is extremely difficult to do this in a way that is cost effective, as you will typically spend more on each ingredient than a bottle of fertilizer that treats several-hundred gallons of saltwater.

Due to this fact, we designed a concentrated version of the fertilizer using the final concentrations from the PDF above. You can purchase that fertilizer here (it's quite good, trust me), or find any number of other fertilizers on the market. We took a lot of care to use Dr. Guillard's concentrations as written, and we can attest to the accuracy of our fertilizer. It is also extremely concentrated, so our 500 mL bottle is enough to fertilize 250 gallons of saltwater. We have successfully raised five species of phytoplankton with this fertilizer (at the time of writing), and know it works well across the board.

If you use our fertilizer, then dose 0.6-2.0 mL of fertilizer for every gallon of culture media (dose to the full volume of water, including the phyto that has been added back to the culture vessel!). If you are measuring in liters, then dose 0.17-0.52 mL of fertilizer for every liter. It is better to under-dose than to overdose in this case, as you don't want to build up excess nutrients in the system.

For diatoms (or other organisms that require silicates), you should also add some silicates to the culture.

6. Repeat weekly.

Once you have your culture going, you should repeat this process every seven days. This gives a nice, dense culture that will be quite nutritious when harvested, and keeps the phyto growing at a fast rate.

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