If you've spent any time around reef-keepers, you've heard someone go on about phytoplankton. It's hyped, often confused with macroalgae or dinoflagellates, and not always understood. Here's the practical, no-fluff version of what it is and why it matters in your tank.
What phytoplankton actually is
Phytoplankton is microscopic, single-celled algae that drifts through the water column. In the ocean, it's responsible for around half of the world's oxygen production and forms the base of nearly every marine food web.
The strains commonly cultured for reef tanks include:
- Nannochloropsis. Hardy, fast-growing, high in EPA omega-3.
- Tetraselmis. Larger cells, broad amino acid profile.
- Isochrysis (T-Iso). Small, golden-brown, high in DHA.
- Pavlova lutheri. Premium feed, rich in DHA, EPA, and sterols.
Different strains feed different organisms. That's why high-quality live phyto products are usually multi-strain blends.
What it does in your tank
Live phytoplankton plays multiple roles:
- Feeds copepods, rotifers, and other microfauna. No phyto, no thriving pod population.
- Feeds filter feeders directly. Clams, sponges, tunicates, feather dusters, and gorgonians all consume live phyto.
- Feeds corals. Particularly LPS, soft corals, and non-photosynthetic species. Even SPS corals show enhanced colour and growth in tanks dosed regularly with phyto.
- Improves water quality. Phyto consumes nitrate and phosphate as it grows.
- Adds oxygen. Photosynthesis happens whether the phyto is in your tank or in a culture vessel.
Live vs preserved phyto
Most of the bottled phyto sold at chain stores is preserved or partially dead. The cells have been killed off with sterilising agents, frozen, or simply allowed to age past viability. They still contain nutrition, but the magic of live phyto is lost.
Live phytoplankton:
- Stays suspended in the water column for hours
- Continues to photosynthesise and produce oxygen until consumed
- Doesn't crash water parameters when overdosed (within reason)
- Triggers feeding responses in corals and filter feeders that dead phyto doesn't
If you can source live phyto, do. It's a different product entirely.
How much to dose
Dosing rates depend on tank size, livestock, and what you're trying to achieve. Rough guide:
- Light dosing for general reef health: 5 to 10 mL per 100 L, every other day.
- Active filter feeders or copepod populations: 10 to 20 mL per 100 L, daily.
- Coral feeding focus: 20 to 30 mL per 100 L, daily, ideally at lights-out.
Start low and watch how your tank responds. You'll know you're dosing enough when copepod populations climb, coral polyps extend longer, and clams open more reliably.
When to dose
Phyto is best dosed in the evening, after the display lights ramp down. Pods do most of their feeding at night, and corals tend to extend polyps for nocturnal feeding too. Dosing in bright light wastes phyto since the cells get blown around the tank without being consumed.
Should you culture your own?
If you go through more than a couple of bottles a month, yes. Home culturing is cheap, sustainable, and gives you fresher phyto than anything you can buy in a bottle. We have separate guides on culturing Nannochloropsis, Tetraselmis, T-Iso, and Pavlova if you want to get started.
The bottom line
Live phytoplankton is one of the few additions you can make to a reef tank that genuinely changes how the system performs. Stronger pod populations, more active corals, healthier filter feeders, and lower nutrient swings. Dose regularly, and your tank will tell you the difference within a few weeks.
