Guide
Aquarium algae: causes and fixes
Every aquarium has some algae. The trouble starts when it grows faster than the plants. The cause is almost always an imbalance between light, CO2 and nutrients, not simply a “dirty” tank.
The most common causes
- Too much light: more than 8 hours a day, a light that is too strong, or sunlight from a window.
- Unstable CO2: plants can't keep up and black beard algae takes the chance.
- Overfeeding and infrequent water changes: leftovers break down into ammonia, which feeds hair algae.
- Too few nutrients for plants: plants stall, algae doesn't.
- A new tank: brown diatoms in the first weeks are normal.
Types of algae and what to do
| Type | What it looks like | Most common cause | What to do |
|---|---|---|---|
| Diatoms | brown film that wipes off easily | a new tank | wipe the glass and wait a few weeks |
| Cyanobacteria | slimy dark green mat, musty smell | weak flow, very low nitrate, decaying leftovers | siphon it out, improve flow, test NO3, 3-day blackout |
| Black beard algae | black or grey tufts on leaves and edges | unstable CO2, weak flow | stabilise CO2, remove affected leaves |
| Green spot algae | hard green dots on the glass | often too little phosphate, strong light | scrape off, test PO4 |
| Hair algae | green threads on plants and decor | too much light, ammonia, uneven dosing | pull out by hand, shorten the light, change water regularly |
| Green water | the whole tank turns green | sunlight, excess light and nutrients | a few days of blackout, a UV-C steriliser |
Step-by-step plan
- Identify the type of algae: each has a different cause.
- Remove what you can by hand: glass, leaves, siphon the substrate.
- Set the light to 6–8 hours a day and move the tank away from the window.
- If you inject CO2, keep it stable throughout the photoperiod.
- Change 30% of the water every week and don't overfeed.
- Add more fast-growing plants: they compete with algae for nutrients.
- Treat algae eaters (shrimp, snails, some fish) as help, not a cure.
Avoid chemicals “just in case”: unless the cause is fixed, algae comes back.
How Aquarium Monitor helps
The algae atlas in the app identifies the type from a few questions or a photo and gives a step-by-step plan: causes, animals that eat it, and spot treatments with warnings for shrimp. Smart plugs keep light and CO2 on a fixed schedule, and test results show the nitrate and phosphate trend.
Frequently asked questions
Is algae harmful to fish?
Most algae isn't. Watch out for cyanobacteria, which can release toxins, and green water, which uses up oxygen at night.
Does a blackout get rid of algae?
For cyanobacteria and green water, yes: usually 3–4 days without light. It doesn't help with black beard or green spot algae, which have other causes.
Why did algae appear after a new light?
A new light is usually stronger. Shorten the photoperiod or dim it and increase it gradually over 2–3 weeks.
Keep an eye on your tank from your phone
The app is free and ad-free. The monitor runs on your Raspberry Pi.