Staghorn algae: how to identify it, what causes it and how to get rid of it

Staghorn algae is a tough, grey-green algae that branches like a deer's antlers. It won't hurt your fish, but it tells you something is off in the tank, most often unstable carbon dioxide (CO2), poor water flow, or too much waste and fertiliser building up.

Pulling it out helps, but it grows back unless you fix the cause. Most algae eaters ignore it, so plan on doing the work yourself.

How to identify it

Colour: grey, grey-green or dull olive. Not bright green.
Shape: thin, stiff strands, usually 1 to 3 cm (0.4 to 1.2 inches) long, that fork into Y-shaped branches like antlers.
Feel: wiry and firmly attached. It doesn't slide off like slime.
Where: leaf edges of slow plants, wood, rocks, filter pipes, heater cords and decorations, and often near the filter outlet where strong flow meets still water.
A simple test: staghorn is a red alga (Compsopogon). If you soak a strand in rubbing alcohol, it often turns reddish.

Look-alikes: black beard algae is darker and grows in short, dense tufts like a paintbrush. Hair algae is bright green, soft and limp. Staghorn sits between the two: grey, branching and stiff.

When it's a problem

Normal-ish: a few strands on a pipe or one leaf. Remove them and keep an eye out.
Early: new strands appearing every week on several surfaces. Act now: it's easy to stop at this stage.
Serious: tufts covering leaf edges and hardscape, slow plants such as Anubias and ferns struggling under it.
Emergency: staghorn itself is never an emergency. If you run CO2 and fish are gasping at the surface, though, your CO2 may be too high: see gasping right away.

What to do right now

Cut off the worst leaves at the base. Slow plants regrow; covered leaves rarely clean up.
Scrub hardscape and equipment. Take out rocks, ornaments and pipes and scrub them with a stiff brush in old tank water.
Do a 30 to 50% water change, and vacuum debris from the gravel, especially in still corners.
Rinse the filter media in removed tank water (never tap water) if it is clogged and flow is weak.
If you inject CO2, check it now. Look at your drop checker (a small indicator that changes colour with CO2 level) and make sure the gas comes on before the lights.
Cut back fertiliser, especially iron-rich products, until you've found the cause.

What to do next

Fix the cause

Sources agree on the common triggers but weigh them differently. Many planted-tank guides blame CO2 first; others say an iron overdose is often the giveaway. Check all of these:

Low or unstable CO2: in high-tech tanks, CO2 that swings up and down during the day is a classic trigger. Turn CO2 on about an hour before the lights and keep it steady. Planted-tank references aim for about 20 to 30 mg/L, checked with a drop checker. Raise CO2 slowly while watching your fish.
Poor flow and dead spots: staghorn loves places where water barely moves. Watch how plants sway. If corners are still, aim the outlet differently, clean the filter, or add a small circulation pump.
Organic waste: overfeeding, dirty filters and rarely vacuumed gravel all raise dissolved waste. Feed less, clean more often and change water weekly. See overfeeding.
Too much iron or fertiliser: if you dose liquid fertiliser, try halving it. Unused iron and trace elements build up when plants can't use them.
Infrequent water changes: nutrients pile up. Change 25 to 50% weekly.

Spot-treat what's left

Liquid carbon (glutaraldehyde products): switch off the filter for 10 to 15 minutes and squirt a small amount from a syringe directly onto the algae. Stay within the label dose for the whole tank. Treated algae often turns white or pink within days, then shrimp and snails eat it. Cautions: Vallisneria, some mosses and some delicate plants can melt, and overdosing can harm shrimp and snails.
Dipping decor out of the tank: scrub, then soak rocks and ornaments (not living plants) in a dilute hydrogen peroxide solution, and rinse very well before putting them back.

Give it two to four weeks. If treated tufts die but new ones keep appearing, the cause is still there.

Algae eaters

Most grazers ignore live staghorn. Young Siamese algae eaters and Amano shrimp may pick at it, and they will clean up algae once spot treatment has killed it. Don't buy fish just for staghorn.

Products that help

Drop checker: shows whether CO2 is in a sensible range, for tanks with CO2 injection.
CO2 solenoid and timer: switches CO2 on before the lights and off with them, for stable levels.
Small circulation pump (powerhead): gets rid of dead spots in larger or heavily planted tanks.
Glutaraldehyde-based liquid carbon: for syringe spot treatment.
Gravel vacuum: removes the waste that fuels it.
Stiff brush and scraper: for hardscape and equipment.

How to prevent it

Keep CO2 steady through the whole light period, or don't inject at all. Low but stable is better than high and swinging.
Make sure every part of the tank has gentle movement.
Clean filters before flow drops off.
Feed lightly and vacuum regularly.
Change 25 to 50% of the water weekly.
Match fertiliser to plant growth.
Keep the lighting period at 6 to 8 hours.

Why it happened

Staghorn grows where plants are under stress and the water carries extra dissolved waste or nutrients. The usual story is a planted tank where CO2 or flow isn't reaching everywhere, or a tank where waste and fertiliser have been building up between water changes.

When to get help

If you've stabilised CO2, improved flow and cut fertiliser for a month and staghorn keeps coming back, ask a local planted-tank club or an experienced aquascaper to look at your setup. Bring photos and your test results.

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