How to Culture Live Phytoplankton at Home
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Live phytoplankton feeds copepods, clams, sponges, feather dusters, corals, filter feeders, and much of the tiny life that helps a reef aquarium thrive.
Culturing phytoplankton at home is not complicated, but cleanliness matters. Most failed cultures can be traced back to contamination, inconsistent care, or waiting too long to split a mature culture.
At Texas Coral Ranch, our process is based on clean saltwater, steady lighting, gentle aeration, proper fertilizer, and careful sterilization.
What You Will Need
- A clean culture vessel
- Freshly mixed saltwater
- A live phytoplankton starter culture
- An air pump
- Flexible and rigid airline tubing
- A light around 6000K to 6500K
- Phytoplankton fertilizer
- Silicate if you are culturing a diatom
- A measuring syringe or graduated cylinder
- Bleach for sterilization
- Sodium thiosulfate for neutralizing bleach
- A clean backup bottle
- Gloves and eye protection
Clear vessels work well because light can reach the culture and you can easily watch its color change.
Prepare the Saltwater
Use freshly mixed saltwater instead of water from an established aquarium. Aquarium water may contain copepods, rotifers, bacteria, ciliates, or other algae that can take over a phytoplankton culture.
We use Instant Ocean mixed to a specific gravity between 1.022 and 1.026. Stability is more important than hitting one exact number every time.
The saltwater, vessel, tubing, and other equipment should be sterilized before the starter culture is added.
Starting a New Culture
We recommend using approximately 8 ounces of live phytoplankton starter for every gallon of prepared culture water.
A simple starter setup uses:
- 3000 milliliters of fresh saltwater
- 8 ounces of live phytoplankton
- Approximately 1.6 milliliters of Part A fertilizer
- Approximately 1.6 milliliters of Part B fertilizer
If you have a 16 ounce bottle, you can use half to start a one gallon culture and keep the other half refrigerated as a backup. A backup gives you another chance if the main culture becomes contaminated or fails to grow.
Keep every phytoplankton strain in its own vessel. When strains are mixed together, one will often outgrow the others.
Add the Fertilizer
We recommend Fritz F/2 two part fertilizer because it provides a more complete nutrient profile for sensitive phytoplankton such as Rhodomonas.
Some essential elements can react and precipitate when stored together in one concentrated solution. Separating the fertilizer into two parts helps keep those nutrients available until they are added to the culture water.
Our recommended dose is:
- 2 milliliters of Part A per gallon
- 2 milliliters of Part B per gallon
Add Part A to the culture water and mix gently. Add Part B afterward. Do not combine the concentrated parts before adding them to the water.
A one part fertilizer may work for hardy phytoplankton such as Nannochloropsis, but we do not recommend relying on it for sensitive strains such as Rhodomonas.
Diatoms such as Chaetoceros, Thalassiosira, and Phaeodactylum also require silicate. Use an 8 percent silicate solution in addition to the regular two part fertilizer. Follow the directions for the silicate product you are using because concentrations and recommended doses can vary.
Add Light and Aeration
Place the vessel under a light around 6000K to 6500K. This is within the daylight spectrum and works well for the species we culture.
A schedule of approximately 16 hours of light followed by 8 hours of darkness is a good place to start.
Use rigid airline tubing to provide steady aeration. The bubbles should keep the cells suspended and the water moving without violently splashing the culture.
Do not tightly seal a vessel that is receiving air. The air must be able to escape.
What Healthy Growth Looks Like
A new culture may look fairly light during its first few days. Its color should gradually become darker as the phytoplankton reproduces.
- Days 1 through 3: The culture may remain light while the cells adjust.
- Days 3 through 5: The color should begin to deepen.
- Days 5 through 7: The culture should become noticeably darker and denser.
This timing is only a general guide. Different species grow at different rates, and temperature, light, fertilizer, and starting density will affect growth.
A healthy culture usually smells like clean seawater. A sour, rotten, or unusually foul smell can indicate contamination or a crash.
When to Split the Culture
Split the culture when it is dark and growing well. In a dense culture, it should become difficult to see light or bubbles clearly through the vessel.
Do not wait for the culture to look stressed. Splitting a strong culture is safer than waiting for maximum density and risking a crash.
- Sterilize the new vessel and equipment.
- Prepare fresh saltwater.
- Add the correct fertilizer.
- Transfer part of the dense culture into the new vessel.
- Save a clean backup bottle.
- Use the remaining culture for feeding or refrigeration.
- Restart gentle aeration.
- Label the vessel with the species and date.
Preventing Contamination
Contamination is one of the most common reasons a culture fails.
Use separate equipment for each strain whenever possible. Never place wet tools from an aquarium, copepod culture, or another phytoplankton vessel into the culture.
When working with copepods and phytoplankton in the same area, handle the phytoplankton first. A copepod can easily travel on wet tubing, tools, containers, or hands and enter the phytoplankton culture.
Do not pour unused phytoplankton back into the culture after it has contacted an aquarium or feeding container.
If a culture becomes slimy, develops clumps, changes to an unexpected color, smells rotten, or suddenly turns clear, it is usually better to restart from a clean backup than to add more fertilizer and hope it recovers.
Sterilizing With Bleach
We use 7.5 percent bleach to sterilize culture water, vessels, and equipment. We then neutralize the bleach with a sodium thiosulfate stock solution.
Wear gloves and eye protection and work in a ventilated area. Never mix bleach with vinegar, acids, ammonia, or other cleaning products. Keep all chemicals labeled and away from children and pets.
Our sodium thiosulfate stock solution contains:
- 10 grams of sodium thiosulfate pentahydrate
- 100 milliliters of reverse osmosis water
Mix until completely dissolved and clearly label the container for bleach neutralization only.
Use the following amounts with 7.5 percent bleach:
| Water volume | 7.5 percent bleach | Thiosulfate stock solution |
|---|---|---|
| 3000 milliliters | 5 milliliters | 5 milliliters |
| 4000 milliliters | 6 milliliters | 6 milliliters |
| 5000 milliliters | 7 milliliters | 7 milliliters |
| 6000 milliliters | 8 milliliters | 8 milliliters |
| 1 gallon | 6 milliliters | 6 milliliters |
| 2 gallons | 11 milliliters | 11 milliliters |
| 3 gallons | 16 milliliters | 16 milliliters |
| 4 gallons | 21 milliliters | 21 milliliters |
| 5 gallons | 26 milliliters | 26 milliliters |
These measurements only apply to 7.5 percent bleach and the stock solution described above. Different bleach strengths or neutralizer concentrations require different amounts.
Sterilization Process
- Fill the vessel with the amount of saltwater being treated.
- Add the correct amount of 7.5 percent bleach from the chart.
- Place the vessel, tubing, and equipment in contact with the treated water.
- Allow everything to sterilize for 2 to 12 hours.
- Add the matching amount of sodium thiosulfate stock solution.
- Allow the chlorine to neutralize completely.
- Confirm that chlorine reads zero with a chlorine test strip before adding phytoplankton.
- Allow any alcohol used on the surrounding work area to evaporate completely.
- Do not introduce wet or dirty equipment after sterilization.
Never add a starter culture until the bleach has been completely neutralized. Residual chlorine can kill the entire culture.
Bottling and Storage
Bottle phytoplankton while the culture is dense, healthy, and free of visible contamination.
Refrigerate bottled phytoplankton promptly. Gently shake or mix it regularly to keep the cells suspended. Do not freeze live phytoplankton.
We recommend using refrigerated phytoplankton within approximately four weeks of the harvest date.
The Basics That Matter Most
- Start with clean equipment and sterile saltwater.
- Use the proper amount of fertilizer.
- Provide steady light and gentle aeration.
- Keep every strain separate.
- Split the culture while it is still healthy.
- Maintain a clean backup.
Phytoplankton is alive and should be treated like livestock rather than a shelf stable supplement. A clean and consistent routine is what keeps a culture productive.
If you need help selecting a strain or setting up your first culture, contact Texas Coral Ranch. We are happy to help you get started.