String algae comes off fastest with a manual removal tool, then prevention through light control and water changes
String algae — also called filamentous algae or hair algae — looks like thin green or brown threads tangled around plants, rocks, and filter intake tubes. It grows in response to excess light and high nutrient levels (especially nitrate and phosphate). The fastest way to remove it is to pull it out by hand or with a tool, then reduce the conditions that let it grow back. Killing it with chemicals works but leaves dead algae in the tank that decays and fouls the water.
Most aquarium keepers find that a combination of manual removal, shorter light hours, and more frequent water changes stops string algae from returning. Some also add fast-growing plants that compete for the same nutrients, or introduce algae-eating fish or shrimp. The method you choose depends on how much algae you have, what lives in your tank, and how much time you want to spend on maintenance.
Key Takeaways
- Pull string algae out by hand or with a toothbrush, old credit card, or algae scraper — this removes it when ready and is the first step in any treatment plan.
- Reduce aquarium light to 6 to 8 hours per day; string algae thrives under 10+ hours and will not return if light stays short.
- Do a 25 to 50 percent water change once or twice per week to lower nitrate and phosphate that feed the algae.
- Fast-growing stem plants like ludwigia or rotala outcompete algae for nutrients and light, preventing regrowth over weeks.
- Algae-eating fish (plecos, Chinese algae eaters) and shrimp (Amano, cherry red) eat string algae but need the right tank size and water conditions to survive long-term.
Manual removal: the fastest first step
Pull the algae out by hand or with a tool before trying anything else. Grab a clump between your fingers and twist it slowly — it will come away from the surface it is attached to. For algae wrapped around plants or in tight spaces, use an old soft toothbrush, a plastic credit card edge, or an aquarium algae scraper. Brush gently so you do not tear plant leaves or scratch the glass.
This method removes algae in minutes and does not add chemicals or dead matter to the water. You will likely see regrowth within a few days if you do not also change the conditions that caused it. But manual removal is still worth doing first because it gives you a clean slate to work from and lets you see which surfaces are most affected.
Reduce light hours to starve the algae
String algae needs light to grow. If your aquarium light is on for 10 or more hours per day, cut it back to 6 to 8 hours. This single change stops most string algae from returning, even if nothing else changes. Algae cannot photosynthesize in darkness, so it will not grow new threads.
If you have live plants that need more light, use a timer to give them the light they need during the day, then turn the light off at night. Most aquarium plants do well on 8 hours of light per day. If you have no plants, 6 hours is enough. Avoid leaving the light on all day or leaving the tank in a window where it gets natural sunlight — both feed algae growth.
Do water changes twice per week to remove nutrients
String algae feeds on nitrate and phosphate that build up in the water. A 25 to 50 percent water change once or twice per week removes these nutrients before algae can use them. This is especially important in tanks with few plants or heavy fish loads, where waste accumulates quickly.
When you change the water, use a gravel vacuum to siphon debris from the bottom of the tank — this removes decaying food and fish waste that turn into nitrate. If your tap water already contains nitrate (common in agricultural areas), a water test kit will show this. In that case, water changes help less, and you will need to rely more on light reduction and fast-growing plants.
Add fast-growing plants to compete with algae
Live plants that grow quickly — such as ludwigia, rotala, water sprite, or stem plants — consume the same nutrients that string algae needs. When these plants are healthy and growing, they shade the algae and use up nitrate and phosphate faster than algae can. Over two to four weeks, the algae will thin out and stop returning.
Plant these species in the substrate or let them float. They need the same light as your fish (6 to 8 hours per day) and benefit from a liquid fertilizer if your tap water is soft or has few minerals. Fast-growing plants also produce oxygen and give fish places to hide, so they improve the tank beyond just fighting algae.
Algae-eating fish and shrimp: what works and what does not
Some aquarium animals eat string algae, but their effectiveness depends on the species, tank size, and how hungry they are. Amano shrimp are the most reliable string algae eaters — they will graze on it constantly and can clear a small tank in weeks. Cherry red shrimp and other small shrimp species also eat algae but more slowly. Both need water between 65 and 75 degrees Fahrenheit and do not do well in tanks with aggressive fish.
Plecos (armored catfish) grow large and eat algae when young, but most species stop eating string algae as they mature and become nocturnal. Chinese algae eaters are aggressive toward other fish and often stop eating algae after a few months. Siamese algae eaters (SAE) eat string algae reliably but are expensive and need a 20-gallon tank minimum. If you add any of these animals, do not rely on them alone — combine them with light reduction and water changes so the algae does not outpace their appetite.
Chemical treatments: when and why to avoid them
Algaecide products kill string algae but leave the dead algae in the tank, where it decays and releases nutrients back into the water. This can cause an ammonia or nitrate spike that harms fish and plants. Chemical treatments also do not address the root cause — excess light or nutrients — so algae often returns within weeks.
If you do use an algaecide, choose one labeled safe for plants and fish, follow the dose on the label exactly, and do a large water change (50 percent) the day after treatment to remove dead algae and chemical residue. Combine it with light reduction and water changes, or it will not prevent regrowth. Most aquarium keepers find that manual removal, light control, and water changes work better and cost less.
Frequently Asked Questions
How long does it take to get rid of string algae?
Manual removal takes minutes. If you also cut light to 6 to 8 hours per day and do water changes twice per week, regrowth usually stops within one to two weeks. If you add fast-growing plants or algae-eating shrimp, it may take three to four weeks for the tank to clear completely. Without addressing light and nutrients, algae will return within days.
Will string algae hurt my fish?
String algae does not poison fish, but heavy growth can clog filter intake tubes and reduce water flow, which stresses fish. It also shades plants and reduces oxygen production. Removing it keeps the tank cleaner and the water quality more stable.
Can I use bleach to kill string algae?
Bleach kills algae but is toxic to fish and plants even in small amounts. Do not use it in an aquarium with fish or plants living in it. If you want to bleach-treat plants or decorations, remove them, soak them in a 1:10 bleach-to-water solution for 15 minutes, rinse thoroughly under running water, and let them dry before putting them back.
What if I have string algae but no plants?
Without plants to compete for nutrients, light reduction becomes even more important. Cut light to 6 hours per day, do water changes twice per week, and consider adding Amano shrimp or a pleco. If you want to add plants later, fast-growing stem plants are the easiest to start with and will help prevent algae from coming back.
Does string algae mean my tank is dirty?
String algae means there is excess light or excess nutrients, not necessarily that the tank is dirty. A tank can be spotless and still grow algae if the light is on too long or if tap water contains high nitrate. Check your light timer and do a water test to see nitrate levels — these are the real causes.