How to Dose Phytoplankton in a Reef Aquarium
Phytoplankton is one of the most useful live foods you can add to a reef aquarium. It supports the small life that makes a reef system feel alive: copepods, feather dusters, sponges, clams, filter feeders, and many corals.
But dosing phytoplankton is not about adding as much as possible. The goal is to add it safely, consistently, and in amounts your aquarium can actually use.
When done correctly, phytoplankton can help build a more biodiverse reef, support copepod populations, and provide natural nutrition throughout the system. When overdone, it can add unnecessary nutrients, cloud the water, or contribute to instability.
This guide walks through a practical approach to dosing phyto in a home reef aquarium.
What Is Phytoplankton?
Phytoplankton are microscopic algae that form the base of many marine food webs. In reef aquariums, live phytoplankton is commonly dosed as a food source for filter-feeding animals and microfauna.
In a reef tank, phytoplankton may be consumed by:
- Copepods and other microcrustaceans
- Feather dusters and tube worms
- Sponges and tunicates
- Clams and scallops
- Some soft corals, LPS corals, and other filter-feeding organisms
Many reefkeepers also dose phyto to help support copepod populations because pods feed on phytoplankton and other fine organic material. A stronger pod population can then become a natural food source for fish such as mandarins, wrasses, gobies, and other small planktivores.
Commercial dosing instructions vary. For example, Seachem’s preserved phytoplankton product suggests 5 mL per 50 gallons twice weekly, while some live phyto hobby guides suggest starting around 1 ounce per 50 gallons of system water. The important point is not to copy the largest number you see online. Start conservatively, observe the tank, and increase gradually.
Why Dose Phytoplankton?
Phytoplankton helps fill a gap in many modern reef systems. Most aquariums are heavily filtered, skimmed, and mechanically cleaned. That keeps water clear, but it can also remove the suspended food particles that naturally feed small organisms.
Dosing phyto can help by:
Supporting copepod populations
Copepods do better when there is a consistent food source. If you are adding pods but not feeding the base of the food web, the population may decline over time.
Feeding filter feeders
Feather dusters, sponges, tunicates, clams, and other filter feeders rely on suspended particles. Phyto gives them access to fine food that fish pellets and frozen foods cannot replace.
Improving biodiversity
A reef aquarium with more microfauna tends to be more stable, natural, and resilient. Phyto is not magic, but it helps fuel that lower level of the ecosystem.
Creating a more natural feeding rhythm
Instead of only feeding large foods to fish and corals, phytoplankton adds small-particle nutrition that moves through the whole system.
How Much Phytoplankton Should You Dose?
The safest answer is: start small and build slowly.
For most reef aquariums, a practical starting point is:
Begin with 5–10 mL per 25 gallons of total system water, 2–3 times per week.
After 2–3 weeks, you can increase slowly if the tank responds well.
For a more established reef with a refugium, copepod population, macroalgae, and filter feeders, you may eventually dose daily. For a newer or lightly stocked tank, lower and less frequent dosing is better.
Simple starting guide
Light dosing:
5 mL per 25 gallons, 2–3 times per week
Moderate dosing:
10 mL per 25 gallons, 3–5 times per week
Heavy biodiversity-focused dosing:
10–20 mL per 25 gallons daily, only if nutrients and water clarity remain stable
Do not jump straight to heavy dosing. A reef tank needs time to adjust.
When Is the Best Time to Dose Phytoplankton?
You can dose phytoplankton during the day or evening, but consistency matters more than the exact time.
For most reef tanks, the best options are:
Option 1: Dose in the evening
Evening dosing works well because many pods and filter feeders become more active after the lights dim. This also gives the phyto time to circulate when fish are less aggressive feeders.
Option 2: Dose when feeding corals
If you already broadcast feed corals, phytoplankton can be added during the same feeding window.
Option 3: Dose into the refugium
For systems with a refugium, dosing phyto into the refugium can help feed copepods and other microfauna where they are protected from fish predation.
A good routine is to dose at roughly the same time each day or each feeding day. Reef systems respond better to steady inputs than random large additions.
Should You Turn Off Pumps or Filtration?
You do not need to shut the whole system down, but you can make dosing more effective by reducing aggressive filtration briefly.
A practical method:
- Shake the phytoplankton bottle gently.
- Turn off the protein skimmer for 15–30 minutes if desired.
- Leave circulation pumps running so the phyto spreads through the system.
- Dose into a high-flow area, refugium, or display tank.
- Turn the skimmer back on after the feeding window.
Do not leave pumps off for long periods. You want the phytoplankton suspended in the water, not settling in one dead spot.
Mechanical filtration such as filter socks, fleece rollers, and fine pads may remove some suspended food over time. If your goal is feeding filter feeders and pods, dosing after a fresh fleece advance or during a lower-filtration window can help more of the phyto stay available.
How to Dose Phytoplankton Step by Step
Step 1: Know your actual water volume
Use total system water volume, not just display tank size. Subtract a little for rock and sand displacement.
A 75-gallon display with a sump may only have 65–75 gallons of actual water depending on rock, sand, and sump level.
Step 2: Start with a conservative amount
For the first two weeks, dose lightly. This lets you see how your tank handles the added food.
Example:
For a 50-gallon system, start with 10–20 mL, 2–3 times per week.
Step 3: Dose into flow
Add phyto near a return outlet, powerhead, refugium inlet, or high-flow area. This helps distribute it throughout the tank.
Step 4: Watch water clarity
The water may look slightly tinted right after dosing. It should clear up. If the tank stays cloudy, reduce the dose.
Step 5: Test nitrate and phosphate
Phytoplankton is food. Even live phyto can affect nutrients depending on dose, culture density, storage quality, and how much gets consumed. Watch nitrate and phosphate trends as you increase. Recent phytoplankton dosing guides commonly recommend increasing gradually while monitoring nitrate and phosphate.
Step 6: Adjust based on results
If the tank looks good, nutrients stay stable, and pods/filter feeders are increasing, you can slowly increase frequency.
Signs You Are Dosing the Right Amount
You are probably dosing appropriately if:
- Water clears within a reasonable time after dosing
- Nitrate and phosphate stay within your target range
- Copepods become more visible at night
- Feather dusters, sponges, and filter feeders appear healthy
- Corals show normal or improved polyp extension
- No film, slime, or cloudiness builds up
The goal is not to see dramatic changes overnight. Phyto works best as a steady, long-term input.
Signs You Are Dosing Too Much
Reduce the dose if you notice:
- Cloudy water that does not clear
- Rising nitrate or phosphate
- Excessive film algae
- Bacterial haze
- Skimmer overflowing after dosing
- Unpleasant smell from the bottle
- Animals closing up after dosing
Overdosing usually happens when reefkeepers try to force results too quickly. Back down, let the system stabilize, and restart at a lower amount.
Live Phytoplankton Storage Tips
Live phytoplankton should usually be stored cold and shaken regularly. Some products have different instructions, so always follow the label.
For live phyto:
- Keep refrigerated unless the product says otherwise
- Shake gently every day or every other day
- Do not freeze
- Do not leave it in hot areas
- Smell it before dosing
- Avoid dosing if it smells rotten, sulfur-like, or foul
Refrigeration slows the culture’s metabolism and helps prevent it from exhausting itself in the bottle.
A healthy bottle should smell marine, earthy, or slightly grassy. It should not smell rotten.
Should You Dose One Phyto Species or a Blend?
Both can work.
A single-species phyto is simple and predictable. A blend can provide a wider nutritional profile because different phytoplankton species vary in size, fatty acid profile, and usefulness to different animals.
For general reef use, a blend can be helpful because it supports a broader range of organisms. For targeted culture work, such as raising pods or larvae, single species may be preferred depending on the animal being cultured.
For a reef aquarium, consistency and freshness matter more than chasing the “perfect” species.
Dosing Phyto for Copepods
If your main goal is building a pod population, dose phyto where pods can actually use it.
Best places to dose for pods:
- Refugium
- Rock rubble zone
- Macroalgae bed
- Low-predation areas
- Display tank after lights out
A good pod-support routine is:
- Dose phyto 3–5 times per week
- Add pods after lights out
- Provide rock rubble or macroalgae habitat
- Avoid relying only on the display if you have heavy pod predators
- Keep dosing consistent after adding pods
Adding copepods once and never feeding them is a common mistake. If you want a sustainable pod population, you need habitat and food.
Dosing Phyto for Corals and Filter Feeders
For corals and filter feeders, broadcast dosing is usually better than target feeding. Phytoplankton is tiny and should move through the water column.
Good candidates that may benefit from phyto availability include:
- Feather dusters
- Sponges
- Clams
- Tunicates
- Some soft corals
- Some LPS corals
- Non-photosynthetic filter feeders
Do not blast phyto directly onto coral tissue with force. Add it gently into flow and let the tank distribute it naturally.
Common Phytoplankton Dosing Mistakes
Dosing too much too soon
Start small. Let the tank adapt.
Not testing nutrients
You cannot manage what you do not track. Watch nitrate and phosphate.
Using old or spoiled phyto
Bad phyto can hurt water quality. Smell and inspect the bottle.
Turning off all flow
Phyto should circulate. Do not let it settle into dead zones.
Expecting instant results
Phyto supports the food web over time. Think weeks, not days.
Adding pods without feeding them
Pods need habitat and food. Phyto helps keep the population going.
A Simple Weekly Dosing Schedule
For a beginner reef tank:
Monday: Dose light phyto amount
Wednesday: Dose light phyto amount
Friday: Dose light phyto amount
Sunday: Observe pods at night and test nutrients if adjusting dose
For a more mature biodiversity-focused reef:
Daily or every other day: Dose small amounts consistently
1–2 times weekly: Check nitrate and phosphate trends
Weekly: Observe refugium and nighttime pod activity
Smaller, consistent doses are usually better than one large weekly dump.
Final Thoughts
Phytoplankton dosing is one of the easiest ways to support the hidden life in a reef aquarium. It helps feed the base of the food web, supports copepods, and provides fine-particle nutrition for filter feeders.
The key is restraint.
Start small. Dose consistently. Watch your water quality. Increase only when the tank shows it can handle more.
A healthy reef is not built by adding more of everything. It is built by creating balance. Phytoplankton can be a powerful part of that balance when used with patience and intention.
