Why algae shows up at all
Algae needs the same three things plants do: light, CO2, and nutrients. It appears when one of those is out of balance relative to what your plants (if any) are using — usually too much light or too many nutrients for the plant mass and CO2 available to consume them.
Identify your algae type first
| Type | Appearance | Likely cause | Typical fix |
|---|---|---|---|
| Brown diatoms | Dusty brown film on glass/substrate | New tank (under 2-3 months), silicates | Usually clears on its own; wipe and wait |
| Green water | Cloudy green, pea-soup water | Free-floating algae bloom, excess light + nutrients | Blackout 3-4 days, UV sterilizer, reduce light |
| Green spot algae | Small hard green dots on glass/leaves | Low phosphate or CO2 relative to light | Balance ferts, reduce light, nerite snails |
| Hair/thread algae | Thin green strands on plants/decor | Excess nutrients, often nitrate/phosphate imbalance | Water changes, reduce feeding, manual removal |
| Black beard algae (BBA) | Dark, bristly tufts on hardscape/leaf edges | Unstable or insufficient CO2 | Stabilize CO2, improve flow, manual removal |
The three levers you're actually adjusting
Light: reduce duration (aim for 6-8 hours) or intensity before assuming you need chemical treatment — this alone resolves a large share of algae issues. Nutrients: excess fish waste, overfeeding, and over-fertilizing all feed algae; match feeding and dosing to your actual plant mass. CO2: in planted tanks, unstable or low CO2 relative to light intensity is one of the most common root causes of black beard algae specifically.
Cleanup crews help, but don't fix the cause
Otocinclus, amano shrimp, nerite snails, and siamese algae eaters all graze different algae types effectively, but adding them without correcting the underlying light or nutrient imbalance just delays the problem — the algae keeps regrowing faster than they can eat it. Use a cleanup crew as ongoing maintenance after you've addressed the cause, not as the fix itself.
Frequently asked questions
What causes brown algae (diatoms) in a new tank?
Brown algae, or diatoms, are extremely common in tanks under 2-3 months old because silicates from new substrate and tap water fuel their growth. It typically clears on its own as the tank matures and usually doesn't need direct treatment.
How do I get rid of green spot algae?
Green spot algae usually signals low phosphate or CO2 relative to light intensity. Reducing light duration/intensity and ensuring balanced fertilization typically resolves it; nerite snails are also effective at grazing it off glass.
What is black beard algae and how do I treat it?
Black beard algae (BBA) is a tough, dark, bristly algae usually linked to unstable or low CO2 alongside organic buildup. Improving CO2 stability, reducing flow dead spots, and manual removal alongside amano shrimp or siamese algae eaters are the standard approach.
Will an algae eater fix my algae problem?
An algae eater (like otocinclus, amano shrimp, or nerite snails) helps manage existing algae but won't solve the underlying light or nutrient imbalance causing it. Fix the cause first, and use a cleanup crew as ongoing maintenance, not a cure.
Does more frequent water changes reduce algae?
It can help with nutrient-driven algae types like green water or hair algae by diluting excess nitrate and phosphate, but it won't fix algae caused primarily by too much light, which needs a lighting change instead.