Algae in a new aquarium: which are normal and what to do

The typical sequence

• a brown film (diatoms) in the first two to four weeks on glass, leaves and rocks – they feed on silicates and usually disappear on their own,
• white slime on wood in the first days – a harmless fungus from the wood that shrimp and snails eat,
• green dust on the glass and hair algae on plants around weeks three to six – when there is more light and ammonia than plants can use,
• later black brush algae or green spots – more often a sign of fluctuating CO₂ or unbalanced fertilizing in an established tank.

Why algae grow

Algae exploit any imbalance between light, CO₂ and nutrients. Plants in a new tank are not growing at full speed yet, the substrate releases ammonia and the light is often set to full. Algae adapt faster than plants.

What helps

• shorten the photoperiod to 6 hours and lower the intensity if needed,
• stable CO₂ for the whole photoperiod, switched on one or two hours before the lights,
• more plants, especially fast growers and floaters,
• more frequent water changes until the tank settles,
• a clean-up crew: otocinclus, Amano shrimp, nerite snails,
• manual removal – pull out threads, trim affected leaves.

Change one thing at a time and give it at least a week. If you change light, fertilizing and CO₂ together you will not know what helped, and the tank cannot settle.

What to avoid

Do not stop fertilizing completely – starved plants grow even slower and algae get ahead. Algaecides treat the symptom, not the cause, and some harm shrimp. Liquid carbon can be spot-dosed on black brush algae, but carefully and within the manufacturer’s dose.

Why keep a journal

Algae usually show up one or two weeks after the change that caused them. With a log of water changes, fertilizer doses, measurements and photoperiod it is much easier to find what changed back then than to guess afterwards.