The short answer
New tanks are the most algae-prone they’ll ever be, because the biology hasn’t stabilised and plants haven’t rooted in yet. You can head off most of it by starting cautiously: keep the photoperiod short, feed lightly, plant heavily from day one, and do regular water changes. Some early algae — especially brown diatoms — is normal and fades as the tank matures, so the goal is prevention and patience, not perfection.
Start with the light low
A young tank can’t yet use much light, so any surplus feeds algae. For the first few weeks:
- Keep your photoperiod to about 6 hours on a timer, extending toward 7–8 only once plants are clearly growing.
- Keep the tank out of direct sunlight, which is uncontrolled light that drives green water and film algae.
- If you have a strong fixture, dim it or raise it while the tank settles. See our planted-tank light picks.
Plant heavily and feed lightly
The single best algae defence in a new tank is lots of healthy plants competing for nutrients from the start.
- Plant heavily on day one, favouring fast growers and floaters that soak up nutrients immediately. Support them with fertilisers.
- Feed sparingly. With few or no fish early on, uneaten food just becomes algae fuel. Feed only what’s cleared in a minute or two.
- Don’t rush stocking. Overstocking a young tank floods it with waste. Add fish gradually.
Expect a settling-in phase
Even done right, most new tanks get a wave of brown diatom algae in the first weeks — it’s harmless and usually clears by itself as the tank matures. Stay consistent and it passes. For more, see how to get rid of brown algae and our how to get rid of aquarium algae guide.