The safe way to add live algae to an aquarium is to start with a small dose, add it where there is water movement, and watch the tank for 24 hours before adding more. Microalgae are harmless to fish in modest amounts, but too much can cloud the water and lower oxygen overnight, when algae stop producing it and begin consuming it.
Before you add
- Check the culture. It should be bright, evenly coloured and fresh-smelling. See how to tell if your algae culture is healthy.
- Match the water. Freshwater species (Chlorella) belong in freshwater tanks. Spirulina grows in alkaline water, so add only small amounts to a neutral tank.
- Know your goal. Are you feeding filter feeders, feeding fry, or seeding a green water culture? Each needs a different amount.
How much to add
There is no single number, but this is a safe progression for a typical aquarium of 50 to 100 litres:
- Start with about 1 mL of dense culture per 10 litres.
- Wait 24 hours. Check clarity, smell and how fish behave.
- If the water is still clear and the filter feeders are active, repeat once a day.
- Increase only if the water clears within a few hours.
A tank with filter-feeding shrimp, snails or Daphnia will take more than one with only fish.
Where and when
Add algae near the filter outflow or an air stone so that cells are quickly distributed instead of settling in one spot. Feed in the morning or midday, never right before lights-out, when algae shift to respiration and use oxygen.
Water quality checks
| Parameter | What to watch | If it goes wrong |
|---|---|---|
| Clarity | Water stays tinted green for more than a day | Stop dosing, do a partial water change |
| Oxygen / fish behaviour | Fish gasping at the surface at night | Add aeration, reduce dose |
| pH | Swings of more than 0.5 between day and night | Increase buffering, reduce algae load |
| Ammonia and nitrite | Above zero | Dead algae are decomposing; water change and filter check |
More on the basics of tank chemistry is in water quality basics in microalgae-fed systems.
Things to avoid
- Do not add algae to a tank under medication without checking the label, since some treatments harm algae and animals alike.
- Do not use brown or foul-smelling culture.
- Do not turn off aeration while the water is green.
Related reading
For filter feeders, see feeding filter feeders in a home tank. If the water turns green on its own, read why aquarium water turns green.
A first-week plan for a new feeding routine
| Day | Dose (per 10 L) | What to check |
|---|---|---|
| 1 | 0.5 mL | Water clarity after one hour and after six hours |
| 2 | 1 mL | Fish and shrimp behaviour; filter flow |
| 3 | 1 mL | Ammonia and nitrite; smell of the tank |
| 4 to 5 | 1 mL | Does the green clear within a few hours? |
| 6 to 7 | 1 to 2 mL if the water clears | Oxygen: fish at the surface before lights on? |
If at any point the water stays green overnight, skip the next dose and let it clear.
Choosing where in the tank
- Planted tank: add in the middle of the day, when plants are photosynthesising and oxygen is highest.
- Sump or filter chamber: a drip near the intake of the filter spreads the algae evenly and the filter pad will catch some, which is fine for a small dose.
- Breeding tub: add by dropper at several points, and keep aeration gentle.
Water changes and algae
Partial water changes remove both waste and the algae in the water. Add algae after a water change rather than before it, or you are throwing away the dose. In tanks that receive algae every day, a weekly change of 20 to 30% keeps nitrate and phosphate from building up, which also reduces the chance of an uncontrolled bloom. The effect on tank chemistry is described in water quality basics.
Combining with medications and treatments
Some treatments, including copper-based products, certain antibiotics and many algaecides, will kill algae, and copper also kills invertebrates such as shrimp and Daphnia. If you must medicate a tank, pause algae dosing and remove any carbon filtration only if the label says so. Do not add algae until the treatment period is over and, if instructed, the medication has been removed by carbon or water changes. UV sterilisers will also remove free algae from circulation.
Troubleshooting overdoses
If you have added too much and the water has turned a deep green, act quickly:
- Increase aeration immediately.
- Do a 30 to 50% water change with dechlorinated water at the same temperature.
- Reduce the light period for a few days.
- Test ammonia and nitrite; repeat the change if they are above zero.
- Skip algae dosing until the water is clear for 24 hours.
Common questions
Can algae harm my fish?
Not in themselves. Problems come from overdosing, which affects oxygen and water quality.
Will it cause an algae bloom?
Adding cultured microalgae does not usually cause a lasting bloom, but excess nutrients and light in the tank can. Keep dosing modest.
Should I turn off the light?
Normal lighting is fine. Avoid extremely long photoperiods that favour wild algae.
Does algae food affect tank algae growth?
Adding cultured algae does not directly cause hair or film algae, but the nutrients that come with it can feed nuisance algae if the tank is already unbalanced.
Can I dose Spirulina to a saltwater tank?
Freshwater Spirulina is not a natural food in saltwater systems and dissolves poorly in salt. Use marine-appropriate products.
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Keep reading
Harvesting Chlorella When Feeding Live Food
Nutrients for Algae When Feeding Live Food
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