Nano Tank Algae Guide

Algae is the number one reason new nano tank owners panic and the number one reason they quit. It should not be either. Every tank has algae — the question is which type, how much, and whether it points to a real problem or just normal biology. I have run planted nano tanks for five years and I have fought every algae on this list; the goal of this guide is to teach you to look at the algae in your tank, name it, and know what to do next without consulting a forum.

Why Algae Happens

Algae is not a disease or a malfunction. It is a primitive photosynthetic organism that takes advantage of the same resources your plants use — light, nitrogen, phosphorus, iron, carbon. When those resources are in balance and your plants are growing fast, the plants outcompete the algae and you see very little of it. When something is out of balance — too much light, too much nutrient, fluctuating CO2, dead spots with no flow — the algae gets a foothold and your plants cannot catch up.

Small tanks amplify this. A 5 gallon nano has the same surface area for algae to grow on as a 20 gallon, with a quarter of the water volume to dilute excess nutrients and a quarter of the plant mass to compete. A single overfeeding event that a 40 gallon tank shrugs off will trigger a hair algae bloom in a 5 gallon. This is not a defect of small tanks — it is a feature. The tank tells you about problems faster. The trade-off is that you have to fix them faster too.

Some algae is genuinely good. A thin green spot algae film on the glass is the sign of a tank with enough light to grow plants. A biofilm on surfaces feeds shrimp and snails. The "zero algae" tank is usually a sterile tank with no plants and a heavy chemical regimen — it looks clean and it is dead. Aim for "controlled algae on surfaces I clean regularly, no algae on plant leaves," not "sterile."

The 6 Common Algae Types

1. Brown Algae (Diatoms)

Brown algae is the dusty brown film that appears on sand, glass, and plant leaves in the first 2–6 weeks of a new tank. It is technically diatoms — single-celled organisms with silica shells — not true algae. The cause is silicates in fresh substrate, fresh water, and fresh filter media. The diatoms bloom, eat the silicates, and then collapse. It wipes off the glass with a magfloat or a sponge, vacuums off the sand easily, and goes away on its own in 4–8 weeks. Patience is the cure. Nerite snails will eat it faster than it can grow.

2. Green Spot Algae (GSA)

Green spot algae looks like hard dark-green dots on the glass, on slow-growing plant leaves (especially Anubias), and on equipment. It does not wipe off easily — you have to scrape it with a razor blade or a credit card. The cause is too much light combined with low phosphate (PO4) or low CO2. In a low-tech tank, the fix is to reduce photoperiod to 6 hours and check your PO4 (you want 1–2 ppm). In a high-tech CO2-injected tank, the fix is to make sure CO2 is stable at 30 ppm — fluctuating CO2 is the most common GSA trigger. A few spots on the glass are normal; GSA covering Anubias leaves means the plant is not growing fast enough to outrun it.

3. Hair / Thread Algae

Hair algae grows in long green filaments attached to plant leaves, substrate, and equipment. It can double in volume in 48 hours and it will smother your plants. The cause is nutrient imbalance — usually too much iron, too much light, or both, combined with too little CO2 or inconsistent CO2. The fix is to manually remove every strand you can (use a toothbrush to twirl it up), reduce photoperiod to 6 hours, and check your iron dosing. Amano shrimp and Siamese algae eaters will eat hair algae but cannot keep up with a bad bloom — manual removal is required first.

4. Black Brush Algae (BBA)

Black brush algae looks like short black or dark-purple tufts on leaf edges, driftwood, and filter intakes. It is technically a red algae (Rhodophyta) despite looking black. It is the most stubborn algae on this list — once it is established, it does not go away easily. The cause is fluctuating CO2 (the CO2 level rising and falling through the day), poor flow (dead spots where organics accumulate), and excess organics in the water column. The fix is to stabilise CO2, increase flow, manually remove the worst-affected leaves, and add Amano shrimp or Siamese algae eaters (the only livestock that reliably touch BBA). Spot-treating with glutaraldehyde (Excel) using a pipette works on small patches.

5. Blue-Green Algae (BGA / Cyanobacteria)

Blue-green algae is not actually algae — it is cyanobacteria, a photosynthetic bacterium. It looks like a slimy blue-green or purple film that coats the substrate, plant leaves, and the glass at the substrate line. It smells earthy, almost swampy. It spreads fast and smothers everything. The cause is low nitrate combined with excess dissolved organics and poor flow — cyanobacteria can fix atmospheric nitrogen, so it outcompetes plants when nitrate is depleted. The reliable fix is erythromycin at half dose (200 mg per 10 gallons) for 5–7 days. It also responds to blackout (3 days of complete darkness) but the erythromycin route is more reliable. Increase nitrate dosing and improve flow to prevent recurrence.

6. Staghorn Algae

Staghorn algae looks like grey-green or black strands that branch like a deer's antlers, attached to plant leaves, driftwood, and filter intakes. It is related to BBA but more diffuse. The cause is low CO2, poor flow, and excess organics — essentially the same triggers as BBA but with more emphasis on organic build-up. The fix is to increase CO2, increase flow, do larger and more frequent water changes, and physically remove the worst strands. Staghorn is the algae of dirty tanks — if you are not gravel-vacuuming and you are overfeeding, this is what shows up.

The Diagnosis Chart

Match what you see to the chart, then jump to the relevant fix. This is the cheat sheet I use myself:

The chart assumes a single algae problem. In reality, a neglected tank often has two or three at once — BBA on the wood, hair algae on the plants, and cyanobacteria at the substrate line. Fix them in this order: cyanobacteria first (because it kills plants fastest and the erythromycin treatment resets the biofilter), then BBA and staghorn (which need flow and CO2 fixes), then hair and green spot (which respond to light and nutrient balance). Brown diatoms you ignore.

The Algae Eater Lineup

Livestock is not the primary fix for algae — the primary fix is identifying and addressing the cause — but a good cleanup crew will keep mild algae in check and let you spend less time scraping. The four I rely on in nano tanks:

Nerite snails (multiple species) are the best glass and hard-surface algae eater in the hobby. They eat green spot, diatoms, and biofilm. They do not reproduce in freshwater (their larvae need brackish water), so you will not have a snail explosion. They are also attractive — zebra, tiger, and horned nerites are all commonly available. Limit 1 per 5 gallons; overstocked nerites will starve.

Amano shrimp (Caridina multidentata) are the best hair algae and BBA eater. They are larger than cherry shrimp (5–7 cm), more aggressive foragers, and they will mow through hair algae, brush algae, and any leftover food. Limit 1 per 5 gallons; they are territorial with each other in small numbers. They do not breed in freshwater.

Otocinclus (Otocinclus affinis and similar) are small suckermouth catfish that eat diatoms and soft biofilm. They are schooling fish — buy 4–6 minimum, never solo — and they need a mature tank with actual biofilm to eat. Wild-caught otos often arrive starved and can be tricky to acclimatise; ask your store how long they have been feeding. They do not eat BBA, hair algae, or green spot.

Bristlenose plecos (Ancistrus sp.) are the heavy-duty algae eater for tanks 20 gallons and up. They eat diatoms, soft algae, and wood biofilm. They stay under 12 cm, they are peaceful, and males grow fleshy "bristles" on the snout. They do not eat BBA, hair algae, or cyanobacteria. Limit 1 per 20 gallons — they have a real bioload and you will need to supplement with zucchini and sinking wafers.

The 4-Step Fix Protocol

For any algae problem that is not cyanobacteria (which needs erythromycin) and not brown diatoms (which needs time), the same 4-step protocol solves 90% of outbreaks:

Step 1: Check the light photoperiod. Reduce to 6 hours a day for low-tech tanks, 8 hours for high-tech with CO2. Use a timer or smart plug — manual switching inevitably drifts. If you have algae and you are running 10+ hours of light, this is your problem and no other fix will work until you address it.

Step 2: Check nutrient balance. Test nitrate (target 10–20 ppm), phosphate (target 1–2 ppm), and iron (target 0.1 ppm). Zero nitrate means your plants are starving and algae is winning; 50+ ppm nitrate means you are overfeeding or under-changing water. Adjust dosing and water change schedule to hit the targets. In a low-tech tank with no dosing, just do 30% weekly water changes and feed sparingly.

Step 3: Manual removal. Scrape the glass, vacuum the sand, pull hair algae by hand with a toothbrush, prune the worst-affected leaves. Algae does not die from water changes — it has to be physically removed from the tank. Throw away plant leaves that are more than 50% covered; they are not recovering.

Step 4: Add a cleanup crew. Nerite snails for glass and biofilm, Amano shrimp for hair and BBA, Otocinclus for diatoms. Add them after steps 1–3 have stabilised the situation; cleanup crews maintain, they do not rescue. A cleanup crew dropped into a tank with severe algae will starve because they cannot eat fast enough.

When Algae Is Actually Good

A completely algae-free tank is usually a sterile tank, and sterile tanks are fragile. A thin green spot algae film on the glass is normal and a sign that your tank has enough light to grow plants. A biofilm on driftwood and plant leaves is normal and feeds shrimp, snails, and otocinclus. The presence of some algae is the presence of a stable microbial community. The goal is not "zero algae" — it is "algae on surfaces I clean, not on my plant leaves."

The exception is shrimp-only tanks and breeding tanks, where a thick coat of algae and biofilm is actively desirable. Shrimp farms run green-water tanks deliberately because the microalgae feeds the shrimplets. A 10 gallon cherry shrimp tank with green spot on the glass, hair algae on the driftwood, and mulm in the substrate is a productive tank. Stop scrubbing it.

The line is: if your fish are healthy, your plants are growing, and you can see into the tank, the algae is fine. If your plants are dying, your fish are gasping, or you cannot see through the glass, the algae is a symptom of a real problem and you need to follow the protocol above. Learn to tell the difference and algae stops being a source of panic.

Frequently Asked Questions