Quick Answer: The best time to dose phytoplankton is 1–2 hours before lights out, when filter feeders are most active and you can safely turn off your protein skimmer. For maximum efficiency, dose directly into a reverse-cycle refugium during its lights-on period so cells stay viable longer before reaching your display tank.
Knowing when should phytoplankton be dosed is just as important as knowing how much to add. Dump it in at the wrong time and your skimmer pulls it out before a single coral polyp gets a meal. Get the timing right, and you’ll see a real difference — better polyp extension, thriving copepod populations, and healthier filter feeders throughout the tank.
What Is Phytoplankton and Why Do Reef Tanks Need It?
Phytoplankton are microscopic, photosynthetic organisms that form the foundation of the marine food web. In the ocean, they feed everything from tiny copepods to large filter feeders. In your reef tank, regular dosing replicates this natural food source for the animals that depend on it.
Common Species Used in the Hobby
Four genera dominate the hobby market, each with a distinct nutritional profile:
- Nannochloropsis (2–5 µm) — High in EPA omega-3 fatty acids, extremely shelf-stable, and the most widely available. A solid all-around choice for most reef tanks.
- Isochrysis / T-Iso (5–6 µm) — Rich in DHA and fucoxanthin. The preferred species for copepod culture and feeding delicate filter feeders.
- Tetraselmis (10–14 µm) — Larger cells with high protein content. Better suited to larger filter feeders like feather dusters and bivalves.
- Chaetoceros (3–8 µm) — A diatom with an exceptional nutritional profile, widely used in aquaculture hatcheries. Requires silicates (1–5 ppm) to thrive.
For most hobbyists, Nannochloropsis is the easiest starting point — it’s available everywhere and works for the widest range of tank inhabitants. If you’re running a copepod culture to support mandarins or seahorses, add Isochrysis to the rotation. For non-photosynthetic (NPS) corals or bivalve systems, a blended product covering multiple species gives the broadest nutritional coverage.
Live, Frozen, or Spray-Dried?
Live or refrigerated phytoplankton — such as Reed Mariculture Phyto-Feast or AlgaGen PhycoPlankton — delivers the highest nutritional value, with a shelf life of 2–6 weeks when kept cold. Frozen phytoplankton is a reasonable backup; cells are often ruptured during freezing, but nutritional value remains useful. Spray-dried and paste forms have the longest shelf life and travel well, but quality varies significantly by brand. If you’re serious about dosing results, live or refrigerated is worth the extra effort.
When Should Phytoplankton Be Dosed? A Timing Guide
This is where most hobbyists go wrong — not with the species or the dose, but with the timing. Phytoplankton cells degrade quickly, get skimmed out, or are destroyed by UV sterilizers. Your goal is to give them the best possible window to be consumed.
Evening Dosing (1–2 Hours Before Lights Out): The Gold Standard
This is the most recommended approach, and for good reason. Most corals and filter feeders extend their feeding tentacles most aggressively in low-light or dark conditions, mimicking natural nighttime plankton availability. Copepods also hit peak feeding activity after dark, so cells get consumed quickly rather than degrading in the water column.
Evening dosing also lets you safely turn off your protein skimmer for 1–2 hours without risking water quality — giving phytoplankton cells a real chance to circulate and get eaten before being pulled out.
Refugium Dosing During Lights-On: The Most Efficient Method
If you run a reverse-cycle refugium (lights on when the display is dark), dosing phytoplankton directly into the refugium during its lit period is arguably the most efficient strategy. Cells can photosynthesize and remain viable much longer, slowly migrating into the display tank via natural water flow. This approach also continuously seeds copepod populations in the refugium, which then spill into the main tank as a live food source.
Automated Split Dosing: Best for High-Demand Systems
For tanks with heavy filter-feeder loads — NPS coral systems, bivalve setups, or active copepod cultures — a single daily dose often isn’t enough. Running a peristaltic dosing pump to deliver 2–4 small doses throughout the day maintains consistent phytoplankton availability and prevents the nutrient spikes that come from a large bolus of cells decomposing at once.
As a practical example: if your daily target is 5 mL per 50 gallons, dose 1.25 mL four times a day rather than all 5 mL at once.
Times to Avoid: Why Morning Dosing Often Wastes Product
Dosing immediately after lights-on is the least efficient approach. Protein skimmers run at peak efficiency in the morning, and phytoplankton cells added to a fully running system get pulled out within minutes. Photodegradation is also higher during peak light hours. You’re essentially feeding your skimmer, not your corals.
| Timing Strategy | Skimmer Impact | Cell Viability | Best For |
|---|---|---|---|
| 1–2 hrs before lights out | Low (can turn off) | High | Most reef tanks |
| Refugium during lights-on | Minimal | Highest | Refugium-based systems |
| Automated split doses | Managed | Consistent | High-demand / NPS systems |
| Immediately after lights-on | High (peak efficiency) | Low | Not recommended |
How Much Phytoplankton to Dose and How Often
Start conservative and observe. A reasonable starting point is 1–5 mL per 50 gallons daily, adjusting based on your tank’s inhabitants and their response.
- 10–30 gallon tanks: 1–2 mL daily
- 30–75 gallon tanks: 2–5 mL daily
- 75–150 gallon tanks: 5–10 mL daily
- 150+ gallon tanks: 10–20 mL daily, or use automated split dosing
A tank full of NPS corals, feather dusters, sponges, and bivalves needs significantly more than a typical mixed reef. Scale up gradually — increase by 1–2 mL per week while monitoring nutrients — rather than jumping straight to a high dose.
Signs You’re Overdosing
- Phosphate climbing above 0.03 ppm (SPS tanks) or 0.1 ppm (mixed reef)
- Nitrate rising unexpectedly
- Cloudy or greenish water that doesn’t clear within a few hours
- Nuisance algae blooms appearing or worsening
Signs You’re Underdosing
- Poor or inconsistent polyp extension in soft corals and LPS
- Feather dusters retracting crowns frequently
- Declining copepod populations in the refugium
- Filter feeders appearing lethargic or closed during feeding periods
Equipment Considerations for Effective Dosing
Protein Skimmers: Turn Them Off After Dosing
Your skimmer is phytoplankton’s biggest enemy. Turn it off — or significantly reduce output — for 1–2 hours after dosing. That’s long enough for cells to circulate and get consumed without causing meaningful water quality decline. Don’t leave it off longer than that.
UV Sterilizers: Dose Downstream or Disable Temporarily
UV sterilizers kill phytoplankton cells on contact. The only workarounds are dosing downstream of the UV unit or turning it off during dosing. This is one of the most commonly overlooked reasons why dosing seems ineffective.
Refugiums: Your Best Tool for Phytoplankton Utilization
A well-set-up refugium transforms how effectively your tank uses phytoplankton. Aim for a refugium that’s 10–20% of total system volume, stocked with macroalgae like Chaetomorpha and seeded with live rock rubble to support copepod populations. Dose phytoplankton directly into this space and let the biology do the work.
Dosing Equipment
- Manual syringe or pipette: Fine for small tanks or infrequent dosing. Simple and inexpensive.
- Peristaltic dosing pump: The best option for consistent daily micro-dosing. Set it and forget it.
- Auto-doser with refrigerated reservoir: Ideal for live phytoplankton in high-demand systems — keeps cells viable between doses.
- Refugium drip system: A slow, continuous drip of diluted phytoplankton into the refugium is highly effective and low-tech.
Keep wavemakers and powerheads running during dosing, and target 20–40 tank volumes per hour of total circulation to keep cells suspended and evenly distributed.
Water Parameters to Monitor When Dosing Phytoplankton
Uneaten phytoplankton decomposes and releases nutrients. Test weekly when you first start dosing and adjust if either nitrate or phosphate trends upward. A reliable phosphate checker makes this easy to track precisely.
| Parameter | Target Range | Phytoplankton Relevance |
|---|---|---|
| Nitrate | < 10 ppm (SPS), < 20 ppm (mixed reef) | Rises with overdosing |
| Phosphate | < 0.03 ppm (SPS), < 0.1 ppm (mixed reef) | Rises with overdosing |
| Temperature | 72–82°F (22–28°C) | Cooler = longer cell viability |
| pH | 8.1–8.3 | Phyto photosynthesis can raise pH |
| Silicate | 1–5 ppm (diatoms only) | Required for Chaetoceros and other diatom species |
When phytoplankton are dosed into a lit refugium, their photosynthesis can cause a temporary pH rise during peak light hours. This is generally beneficial for reef chemistry, but worth monitoring if your pH is already at the high end of the normal range.
If you’re dosing diatom species like Chaetoceros, check your silicate levels. Most reef tanks run very low silicates by design — you may need to supplement to support diatom cell wall construction.
Frequently Asked Questions About When to Dose Phytoplankton
How often should I dose phytoplankton in a reef tank?
Daily dosing is ideal for most reef tanks, especially those with active filter feeders or copepod cultures. Every other day works for lightly stocked systems. High-demand tanks — NPS corals, bivalves, mandarin support systems — benefit most from automated dosing 2–4 times per day.
Should I turn off my protein skimmer when dosing phytoplankton?
Yes, for 1–2 hours after dosing. Skimmers remove phytoplankton cells from the water column before corals and filter feeders can consume them. Turning it off briefly after dosing dramatically improves how much phytoplankton actually reaches its intended consumers.
Can you overdose phytoplankton in a reef aquarium?
Absolutely. Excess phytoplankton decomposes and releases nitrate and phosphate, which can fuel nuisance algae and stress SPS corals. Signs of overdosing include cloudy water, rising nutrients, and increased algae growth. Always start with a conservative dose and scale up slowly while monitoring parameters.
Is it better to dose phytoplankton during the day or at night?
Night — or more precisely, 1–2 hours before lights out — is better for most display tanks. Filter feeders are more active in low-light conditions, skimmers can be safely turned down, and photodegradation is reduced. The exception is refugium dosing, where adding phytoplankton during the refugium’s lit period allows cells to photosynthesize and stay viable longer.
Can I dose phytoplankton directly into a refugium?
Yes, and for many systems this is the most effective approach. Dosing into a refugium running a reverse photoperiod gives phytoplankton cells maximum contact time with copepods and other microfauna before water flows into the display tank. It also bypasses the display skimmer entirely, so more cells actually get consumed.
