Algae ID Guide: Hair Algae, Bubble Algae, Diatoms, Cyano & Dinoflagellates

Almost every reef tank goes through an algae phase at some point, and the fix is completely different depending on what you’re actually looking at. Green hair algae, bubble algae, diatoms, cyanobacteria, and dinoflagellates all get lumped together as “algae” by frustrated beginners, but they’re different organisms with different causes and different treatments — and using the wrong approach can waste weeks while the real cause keeps feeding the bloom.

Green Hair Algae (GHA)

Green hair algae grows in soft, wavy strands or tufts across rock, sand, and equipment, and it’s usually the first nuisance algae a new reef keeper meets. It’s driven by excess nitrate and phosphate combined with lighting that’s feeding algae more than it’s feeding your corals. Manual removal works well here — pinch clumps between your fingers and immediately siphon them out before loose fragments can resettle and regrow elsewhere. Because it’s soft and palatable, a solid herbivore cleanup crew will keep grazing it down, but without fixing the underlying nutrient levels, it will keep coming back no matter how many times you pull it or how many snails you add.

Bubble Algae (Valonia)

Bubble algae shows up as small, shiny green spheres stuck to rock, usually tucked into crevices. The one rule that matters most: never crush it in the tank. Popping a bubble releases reproductive cells directly into your water, which is how one bubble turns into dozens. Remove it by twisting and detaching at the base rather than pulling from the top, and lift the freed bubbles out with a net or siphon before they can rupture. Emerald crabs are the standard biological control — they puncture bubbles at the base and consume them in a way that keeps the reproductive cells contained — but a well-fed crab in a tank full of prepared food may not bother, so don’t rely on one alone if you already have a real infestation.

Diatoms (Brown Algae)

Diatoms coat the sand, glass, and rock in a fine brown or golden-brown dusting, and they’re most common in a tank that’s only a few weeks to months old. They feed on silicate, which is often present in tap water and gets used up as a new tank matures — this is one of the few algae issues that frequently resolves on its own within a couple of months as silicate runs out and other biology establishes. If it lingers, switching to RODI water (which strips silicate along with everything else) is the real fix, since regular water changes with silicate-laden tap water just keep resupplying the problem. Turbo and nerite snails are efficient diatom grazers and are a great early addition to a new tank’s cleanup crew.

Cyanobacteria (Red Slime)

Despite the name, cyanobacteria isn’t a true algae — it’s photosynthetic bacteria, usually showing up as a red, purple, or occasionally green slimy mat that sheets over sand and rock. It’s typically driven by a combination of low flow (dead spots where detritus settles), inconsistent water changes, and nutrient imbalance in either direction — both very high and very low nitrate/phosphate can trigger it. Unlike hair algae, it has no real attachment to the surface, so it lifts off easily with a turkey baster or siphon during manual removal — just be thorough, since leaving any behind lets it recolonize fast. Improving flow to eliminate dead spots and getting back on a consistent water change schedule usually resolves it; a product like Chemiclean is available for stubborn cases but works best paired with fixing the underlying flow and nutrient issues, not as a standalone fix.

Dinoflagellates (“Dinos”)

Dinoflagellates are the most frustrating of the group — a thin, often bubbly or snot-like brown or gold film that can appear similar to diatoms or cyano at first glance but behaves very differently and is far harder to eliminate. Under a microscope you’ll see individual cells actively swimming rather than sitting still, which is the clearest way to confirm what you’re dealing with. Some species produce genuine toxins, so handle an outbreak with gloves and avoid skin contact. The most effective approach combines a 3–4 day full blackout (lights off, tank covered, refugium light off too, but skimmer and any UV sterilizer left running) with nightly dosing of food-grade 3% hydrogen peroxide at roughly 1 mL per 10 gallons, dosed after lights-out since peroxide breaks down in light. A UV sterilizer run continuously helps knock down the free-swimming population alongside the blackout. None of this sticks if the underlying nutrient imbalance and low microbial diversity that let dinos take over in the first place aren’t also addressed — a dino bloom that clears with blackout alone often returns within days.

Clean-Up Crew by Algae Type

• Green hair algae: turbo snails, astrea snails, sea hares for severe cases
• Bubble algae: emerald crabs (as a supplement to manual removal, not a solo fix)
• Diatoms: turbo snails, nerite snails
• Cyanobacteria: a healthy, diverse pod population helps, but manual removal and flow/nutrient fixes do the real work — CUC alone won’t clear it
• Dinoflagellates: CUC generally won’t touch it — this one is a water-chemistry and light problem, not a grazing problem

How to Avoid Algae in the First Place

Almost every algae problem traces back to the same three levers: excess nutrients, lighting that outpaces what your corals can use, and an undersized or unbalanced cleanup crew. Keep nitrate and phosphate in range for what you’re keeping, run RODI water to keep silicate and phosphate out of every water change, keep flow strong enough that nothing settles into dead spots, and don’t run your photoperiod longer than your corals actually need — 8–10 hours is plenty for most systems. Build your cleanup crew proactively rather than reactively, and stay consistent with water changes and testing so a small imbalance gets caught before it becomes a bloom.

Dealing With an Active Outbreak

Identify what you’re actually looking at first — the treatment for cyano will do nothing for dinoflagellates, and vice versa. Start manual removal immediately regardless of type; it won’t solve the root cause on its own, but it buys time and reduces the population you’re fighting. Then work the actual cause: test and correct nitrate and phosphate, check your flow for dead spots, confirm your RODI unit is still producing zero TDS water, and reassess your photoperiod. Give any fix 2–3 weeks to show results before concluding it isn’t working — algae blooms build up over time and clear the same way.

The Bottom Line

Correctly identifying what kind of algae you’re fighting is half the battle — hair algae, bubble algae, diatoms, cyano, and dinoflagellates all need different tools. Combine manual removal with the right cleanup crew and a real fix to the underlying nutrient, flow, or lighting cause, and most outbreaks clear within a few weeks. Not sure what you’re looking at? Reach out with a photo, or browse our current stock for cleanup crew additions.

Algae Is a Symptom, Not the Disease

Whatever type you’re fighting, treat the outbreak and the underlying nutrient source at the same time — scrubbing algae off rock without fixing the phosphate or nitrate feeding it just buys you a few weeks before it’s back. Test your source water for phosphate (tap and RO/DI water are common silent sources), check your feeding and skimmer output, and confirm you’re not overdue on water changes before assuming you need a more aggressive product-based fix.