𝐂𝐡𝐨𝐨𝐬𝐢𝐧𝐠 𝐭𝐡𝐞 𝐑𝐢𝐠𝐡𝐭 𝐏𝐡𝐲𝐭𝐨𝐩𝐥𝐚𝐧𝐤𝐭𝐨𝐧 𝐟𝐨𝐫 𝐘𝐨𝐮𝐫 𝐑𝐞𝐞𝐟 𝐓𝐚𝐧𝐤:

𝐀 𝐏𝐫𝐚𝐜𝐭𝐢𝐜𝐚𝐥 𝐆𝐮𝐢𝐝𝐞

As a reminder, I write these articles to help you avoid the loops I got stuck in. If I missed a nuance you’ve wrestled with, let me know. We’re all learning. I have loved phytoplankton for 25 years. I have cultured phytoplankton professionally and for fun and I hope this guide will help you on your journey to an even better reef tank.

-Tamara

This is the second article in my new Phytoplankton series stay tuned for future articles when we dive deeper into culturing techniques.

Article 1: How to Dose Phytoplankton

Article 2: (This one) How to Select the Correct Phytoplankton for Your Reef

Introduction

If you've ever stood in front of a fridge full of green bottles, or browsed an online store with a dozen different "phyto" options, and felt that familiar reefing paralysis—Which one is best?—this guide is for you.

Selecting phytoplankton isn't just about picking the greenest water. It’s about matching a microscopic powerhouse to the specific needs of your tank’s inhabitants. Get it right, and you fuel a thriving, invisible food web. Get it wrong, and you might as well be dumping expensive pond water and possibly crashing your tank.

In the previous article, we built a framework for dosing. Now, we’re going back a step—to the selection. This is Part 2 of our phytoplankton series, and we’re tackling how to choose your strains, how to spot a healthy culture, and why this choice matters more than you might think.

Reader Promise: This isn't a biology lecture. It's a practical, strain-by-strain breakdown of the "who eats what" in your tank. You'll learn how to read a product label, what to look for in a live culture, and how to make an informed choice that aligns with your reefing goals. My goal is to turn that wall of green bottles from a confusing array into a simple, strategic decision.

Phytoplankton 101: It’s Not All Green Water

Let's break down what these strains are. They are not generic "plant plankton," but as different types of superfoods for your tank’s cleanup crew and filter feeders.

• Nannochloropsis is hardy, packed with basic nutrition, and a good all-rounder. This is also the easiest to culture so I will be covering that in the next article in the series.

• Tetraselmis is motile (swims), and eagerly consumed by everything from copepods to brine shrimp. In the goldilocks scale, this algae is right in the middle with both nutrience and difficulty in growing.

• Isochrysis (T-Iso) is smaller, richer in healthy fats (HUFA's), and a premium food for sensitive larvae and clams. I think of this algae as the gold standard. It isn't the easiest to culture but everything seems to love it.

• Rhodomonas is a niche, potent option packed with pigments and nutrition, loved by picky eaters. I struggled with my blueberry gorgonian for years until I started dosing Rhodomonas and then it finally started thriving.

The strain you choose determines who benefits most in your tank. Or if you are going to culture it might also depend on your skill level and the time you have available.

The Selection Matrix: Matching Strain to Goal

Tamara’s Pro-Tip: When in doubt, or for a balanced approach, a high-quality blend is often the best choice. It’s like offering a well-rounded diet instead of just one vegetable.

The Health Inspection: How to Spot a Healthy Culture (Before You Buy or Use)

A bottle of dead or dying phytoplankton is worse than useless—it’s a pollutant. Here’s how to be a phyto detective.

At the Store (or When the Package Arrives):

1. Check the Dates: Look for a "Harvest Date" or "Best By" date. Live phytoplankton has a shelf life. Anything over 4-6 weeks old is a gamble.

2. Look at the Color: It should be a vibrant, consistent green (or golden-brown for Iso/Tetra). Avoid bottles with:

   • A dark, sludgy layer at the bottom with clear water on top. 

   • A yellowish, "sickly" hue.

   • Any signs of white, fuzzy contamination (this is mold/bacteria).

The At-Home Sniff Test (The Ultimate Test):

Unscrew the cap and take a cautious whiff.

• GOOD: Smells like the ocean, fresh seaweed, or even slightly sweet and "grassy." A healthy, live culture has a clean, pleasant marine smell.

• BAD: Smells sour, rancid, like rotten eggs, or like a stagnant pond. You'll know the rotten egg smell when you smell it - this culture has crashed and is decomposing. Do not dose it into your tank.

The Settling Test:

Gently shake the bottle and let it sit upright for 30 minutes.

• Healthy: Will show a slight gradient, but the water column will still be tinted. Cells settle slowly. If it just settles back out quickly I would be wary.

  • Aerated: When Phyto is not aerated (not all species are motile) or when

  • Chilled: When phyto is refrigerated it will settle as it goes into a dormant state, but if you shake it, it should stay in the water column. If it just settles back out quickly I would be wary.

• Unhealthy/Old: Cells will sink rapidly to the bottom, leaving crystal-clear water above. This indicates low vitality or a preservative-based product.

Live vs. Concentrated vs. Paste: What Are You Really Buying?

Live Phytoplankton: The gold standard. Living, reproducing cells. They will multiply in your tank if conditions (short term) are right, providing sustained nutrition. I like to think about it like they are cleaning your tank until consumed by removing nitrates and phosphates (very minimal but every bit counts!).

• Best for: Seeding tanks, feeding pods, and direct consumption by filter feeders.

• Everyone seems to grow and sell their own phytoplankton these days. It doesn't mean they know what they are doing or that you are purchasing a quality product. It is okay 

Concentrated Live: Often a live culture that has been centrifuged or concentrated. Viability varies wildly by brand and age. Ask about the concentration process—gentle methods preserve cell health. There are wonderful reputable sources out there that will have clearly marked bottles.

Preserved (Paste/Dead): The cells are no longer alive. The nutrition is largely intact, but it will not seed your tank or reproduce. This is often the best bang for your buck unless you have a local live source or grow your own.

• Best for: Enriching rotifers or brine shrimp in a separate culture. It's a food, not an ecosystem builder.

• Be very careful dosing this to your tank as it can decompose in your system if not eaten or removed by your filtration.

Tamara's Pro Tip: For dosing your display tank, fresh, live phyto is almost always the most effective choice. You're paying for life, not just nutrients.

Frequently Asked Questions

Q: Can I mix different strains in my tank?

A: Absolutely! In fact, it's encouraged. A diverse phytoplankton diet supports a more diverse and resilient microfauna population.

Q: How important is cell count (cells/mL)?

A: It's a useful metric for comparing similar products, but freshness trumps a high cell count every time. A bottle with 2 billion cells/mL of dead algae is inferior to a bottle with 1 billion cells/mL of vibrant, live algae.

Q: I have a UV sterilizer. Should I turn it off?

A: Yes. If your UV is always on, you are essentially feeding "dead" phyto to your tank. In this case, you might prioritize nutritional content (Isochrysis, Rhodomonas) over the benefits of "liveliness." However, the best practice is to turn the UV off for a few hours after dosing, as discussed in our dosing article.

Q: What’s the one mistake you see most when people choose phyto?

A: Buying the largest, cheapest bottle without checking the date or understanding the strain. They end up with a gallon of low-quality, generic "green water" that does little for their specific tank goals.

Conclusion: Choose with Intention

Selecting phytoplankton isn't a chore—it's your first act of intentional feeding. It’s the decision that determines what fuels the very base of your tank’s food pyramid.

Start with your goal. Let it guide you to the right strain or blend. Then, become a quality detective—check dates, inspect colors, and always, always do the sniff test. Whether you buy it or culture it yourself, prioritize vibrant, live cells.

When you match the right, healthy phytoplankton to your tank’s needs, you’re not just adding a supplement. You’re cultivating life from the bottom up. And that’s where the most resilient and beautiful reefs are built.

Next Up in the Series: Part 3 – How to Culture Phytoplankton at Home: A Simple, No-Fuss Guide.

Happy Reefing!