The short answer
Scrape the walls with a tool made for your tank (glass or acrylic), scrub decor and twirl off hair algae, then siphon the loose algae out in a partial water change. To keep it away, test tank and tap water for nitrate and phosphate, put the light on a timer, avoid direct sun and feed sparingly.
Aqueon, an aquarium equipment maker, says “Some algae growth in a mature aquarium is normal”. To work out which algae you have and what is feeding it, start with our explainer on why an aquarium grows algae.
These steps are written for freshwater tanks. More guides to common tank problems are in our Water Problems section.
Warning
If your fish look unwell, test the water and contact an aquatic veterinarian. This guide covers algae, water and equipment; it is not a diagnosis or treatment for sick fish.
What you need
- A scraper or pad that suits your tank’s walls. Dr. Jessie Sanders, an aquatic veterinarian, writes: “It is critical that you use a scrubber that is appropriate for your tank type. Glass scrubs are more abrasive and can scratch acrylic, causing a hazy appearance. If you are unsure, it is better to choose one for acrylic.” Aquarium Co-Op, an aquarium retailer, says its melamine algae scrubber “won’t scratch your glass and acrylic tanks”.
- A toothbrush kept for the tank, plus plant scissors if you keep live plants.
- A gravel vacuum, a bucket and a tap water conditioner for the water change. Our water conditioner roundup compares conditioners.
- Nitrate and phosphate tests. Our freshwater test kit roundup compares kits for the nitrate test. For phosphate, API, a water-care and test-kit maker, sells a separate Phosphate Test Kit that it says reads “from 0 – 10.0 ppm”.
The three tools below are picks from our algae scraper roundup, which sorts scrapers by glass and acrylic. By each maker’s description: Mag-Float lists the Medium (125) model under “FOR GLASS AQUARIUMS”, “Recommend for aquariums up to 125 gallons”, for glass up to 3/8 inch thick, with its scraper blade sold separately; the API scraper is “For glass aquariums only”; and Continuum Aquatics calls its cyan blade “softer than acrylic”. Two are glass-only; the Continuum is the one sold for acrylic.
| # | Preview | Product | Price | Link |
|---|---|---|---|---|
| 1 |  | Mag-Float Medium Glass Floating Algae Scraper 125…Floating magnet for glass | $33.89 | Check on Amazon |
| 2 |  | Continuum Aquatics AquaBlade P – Acrylic Safe Aquarium…Hand scraper for acrylic | $32.98 Prime | Check on Amazon |
| 3 |  | API Algae Scraper for Glass Aquariums 1-Count ContainerGlass-only scraper | $8.99 Prime | Check on Amazon |
Clean it out: eight steps
Test the tank and your tap water first
Aqueon’s advice: “If you start to see excessive algae growth, test your aquarium and tap water for nitrate and phosphate.” LiveAquaria, an aquarium livestock and supply retailer, says that if the tap shows “significant levels of phosphate, nitrate, or iron, your battle against algae will be futile until levels are reduced.” If nitrate reads high, our guide to lowering nitrates covers it.
Unplug the filter and heater before your hands go in
Fluval, an aquarium equipment maker, cautions in its AquaClear filter manual: “Always unplug or disconnect all appliances in the aquarium from electricity supply before placing hands in water, before putting on or taking off parts and while the equipment is being installed, maintained or handled.” Hydor, a heater maker, gives two waits. Its Theo manual says: “To avoid possible breakages due to thermal shock, the heater should be disconnected from the power supply at least 5 minutes before taking it out of the water or before adding or changing the water.” Its support page says “at least 30 minutes before any waterchange.” The two differ, so follow your own heater’s manual.
Scrape the walls with the tool made for them
API, for its glass scraper: “Do not use on acrylic, Plexiglas, or any surface other than glass. Do not use on dry surfaces.” and “Caution: If aquarium gravel is picked up on pad, wash off before using since gravel will scratch glass surfaces. Do not use pressure so extreme that scratching or breaking of aquarium glass occurs.” Continuum, for its acrylic-safe blade: “Acrylic safe blade is necessarily soft and can therefore pick up sand or small bits of gravel if shoved too hard down into the substrate.” It says to check the blade by running a finger or thumbnail across it, warns that some tanks are made from low grade acrylic “that can be easily scratched”, and adds: “User must exercise proper judgement in use and should test blade in an inconspicuous area prior to use to insure compatibility.” For stubborn green spot algae on glass, Aquarium Co-Op suggests scraper blades it calls “glass-safe”.
Scrub decor and hardscape, and twirl off hair algae
Aquarium Co-Op suggests a toothbrush for decorations, hardscape and even plant leaves; for certain hair algae types, it suggests “twisting the toothbrush so that the algae winds up like spaghetti on a fork.” For synthetic decor you can lift out, Dr. Sanders says: “Take all your décor to the sink and scrub it with hot water and a designated toothbrush. For caked on algae, you can soak your décor in hot, not boiling water, for a few minutes. Do NOT use any detergent!” Aqueon adds: “Don’t use buckets, scrub pads or any other equipment on your aquarium that have been used with soap or other chemicals.”
Trim leaves covered in filamentous algae
Aqueon: “Plant leaves that become covered in filamentous algae should be trimmed out.”
Clean the filter while the algae settles, unless the tank is new
Aqueon’s order: “While the scrubbed algae is settling to the bottom, remove the filter and take it to a sink to be cleaned.” Fluval’s 07 canister manual: “Rinse biological media with aquarium water, never tap water, or replace as required. Never replace all biological media at one time, as valuable colonies of beneficial bacteria would be lost.” In a newly set up tank, skip this step: Aqueon’s new-tank page, which speaks of a filter’s “first 30 days”, says “Cleaning a brand new filter or replacing the cartridge or media does nothing good, and potentially eliminates the good bacteria that are trying to get established.”
Siphon out the loose algae in a partial water change
Aqueon’s checklist: “Using your gravel vacuum, do a water change of at least 25% of the aquarium. The siphon will now be able to suck up the loose algae that has settled to the bottom.” Its do’s and don’ts page advises against more: “Avoid water changes of more than 25% as larger water changes can stress your fish and upset the balance in your aquarium.” Dr. Sanders sets a looser cap: “Do not remove more than 50% of the tank water at a time to avoid sudden swings in temperature and pH.” 25% is the one amount that fits both Aqueon pages and stays inside Dr. Sanders’ limit (our reading). LiveAquaria warns, “do not leave loose algae floating about in your aquarium where it will decay, increasing the amount of dissolved organic material and lowering the oxygen level.” Hydor’s Theo manual: “Always ensure the water level stays above the ‘Minimum water level’ mark shown on the heater”. Your fish can stay in; Dr. Sanders says to “keep an eye on them and don’t suck them up!”
Refill with treated water at tank temperature, put the filter back, then switch back on
Aqueon: “After a good rinse, replace all media into the filter and reinstall to the aquarium but do not plug back in yet.” Then it says to “add water conditioner to new water and then refill the aquarium. Be sure to match temperatures of the new water with the aquarium.” Dr. Sanders asks for a dechlorinator “that treats chlorine AND chloramine”. Fluval’s AquaClear manual: “Do not allow filter pump to run dry.” and “Follow operating instructions to restart and prime power filter.” Hydor’s support page says its heater “can ONLY then be turned back on when the Aquarium is full and the heater is completely submerged.” Our guide to cleaning a fish tank covers the full routine.
Keeping algae out after the clean
LiveAquaria: “Manual cleaning won’t prevent the return of algae, but it helps make other management methods more effective.”
| Habit | What the source says | Source |
|---|---|---|
| Light hours | 6-8 hours at most unplanted; 10-12 hours planted | Aqueon |
| New planted tank | Start at 6-8 hours; raise toward 8-12 as plants grow | Aquarium Co-Op |
| Sunlight | No direct sun, “even if it’s only for a few hours a day” | Aqueon |
| Feeding | What fish eat “in under 2 minutes” | Aqueon |
| Water changes | 10% a week or 25% every two weeks | Aqueon |
| Nitrate, phosphate | Below 10 ppm and below 0.5 ppm when algae is excessive | Aqueon (tank type not stated) |
Sources differ on a safe nitrate level: TFH, a fishkeeping magazine, reports expert recommendations that “range from under 25 ppm to 80 ppm”. For planted tanks the two light figures also differ: Aqueon gives 10-12 hours, while Aquarium Co-Op starts a new planted tank lower and adjusts as the plants grow.
Light: hours, brightness and the sun
Aqueon recommends a timer, and warns that a light left on 24/7 “not only stresses your fish but can lead to excessive algae growth.” Aquarium Co-Op’s rule for planted tanks is “do not leave the light on all night, not even in blue moonlight mode”, and if algae spreads, “decrease the duration again.” Our planted aquarium light roundup compares lights by tank length. For a tank near a window, Aqueon suggests blinds or curtains closed during the day.
Tap water and phosphate
Excess nutrients are the other fuel. Laura Muha, in a TFH column about her own fish-only tank with live rock, notes that “many municipalities add them to the water supply to prevent pipe corrosion”, meaning phosphates, and that “Like nitrates, phosphates are best removed via water changes, but you can also buy phosphate-adsorbing media for your filter.” API adds that phosphate enters the tank from “decaying organic matter, such as dead algae and uneaten fish food”. If your tap water itself carries nutrients, Aqueon suggests reverse osmosis or deionized water for changes. Our phosphate remover roundup compares media and liquids.
Plants, temperature and flow
Aqueon says plants “work best at preventing algae when the aquarium is heavily planted”, naming fast-growing stems such as “hornwort, wisteria and teardrop Rotala”; it adds they “should not be placed in aquariums containing herbivorous fish or those that dig excessively.” Muha writes that “adjusting the temperature downward a couple of degrees (as long as it remains in the appropriate range for your fish) can sometimes slow an algae outbreak”.
Algae eaters, with their limits
Aquarium Co-Op ranks algae eaters second to hand cleaning because “each algae-eating species only eats certain kinds of algae”. Buce Plant, an aquarium plant retailer, prefers shrimp and snails for a planted aquarium and does not recommend plecos in severe cases, “as they will end up adding to the waste, potentially worsening the algae.” Aqueon says to “Use caution when adding plecostomus to planted aquariums, as some species are known to eat broadleaf plants like Amazon swords and Anubias.”
Products, blackouts and other quick fixes
Aqueon says chemical algaecides “should never be the first choice when dealing with algae problems, as they do not address the original cause of most outbreaks”, and “should always be used as a last resort.” Its instruction: “Always follow product directions and remove carbon and other chemical media from your filter before using algae control chemicals.” Muha leaves them out even though most such products are technically “fish safe”, in her words, because “anything you add to your water causes some level of osmotic stress for your fish.”
Warning
Read the exact product’s label before you add it. API’s AlgaeFix, sold for freshwater aquariums, says: “Do not overdose.” “Caution: do not use with freshwater crustaceans, including shrimp, crabs, and lobsters.” “Siphon or scrape any dead algae from aquarium.” “It is a violation of federal law to use this product in a manner inconsistent with its labeling.” If you keep shrimp, that label rules it out.
For green water, Aqueon warns that “sudden algae die-offs can spike ammonia and nitrite levels and some coagulants interfere with the fishes’ gills.” Test ammonia and nitrite after any algaecide or blackout (our suggestion); for a reading above zero, our guides to lowering ammonia and lowering nitrite cover what to do.
For green water, the suspended algae of a green bloom, Aqueon calls diatom filters and UV sterilizers “very effective at solving green water blooms, but not other forms of algae.” With a UV sterilizer, Aqueon adds, “partial water changes should be done every few days to prevent ammonia and nitrite build-up.” It says blacking out the tank under a blanket for several days “can be effective, but this is detrimental to live plants and may result in ammonia and nitrite spikes when the bloom suddenly dies off.” Our green water guide has more.
Mistakes and troubleshooting
- The glass is green again within days. Retest tank and tap water and work through the habits table above, one change at a time: Muha, describing her own tank, held back because “if I fixed everything all at once, I’d never know which factors were contributing most.”
- Fine scratches or haze on acrylic. That can be the glass-scrub damage Dr. Sanders describes; switch to an acrylic-safe pad or blade.
- Green water returns after every water change. Aqueon says the effect of water changes on a bloom is “temporary and the problem quickly returns, often with a vengeance due to the addition of nutrients from tap water.” See the green water section above.
- Brown dust in a new tank. Aqueon says these diatoms “will usually die off on their own after the tank cycles.” Our guide to brown stuff in an aquarium has more.
Ready to buy?
Our algae scraper roundup compares magnetic and handheld scrapers for glass and acrylic, our freshwater test kit roundup compares kits for the nitrate test, and our Cleaning & Maintenance section gathers more cleaning gear. Snails grazing the glass? Our guide to getting rid of snails in an aquarium covers removal without chemicals.
Frequently Asked Questions
Q. How do I get rid of algae in my fish tank fast?
Scraping, scrubbing and siphoning clear the visible algae in one session. Aqueon says algaecides “can provide a temporary solution, but sooner or later the problem will return if the original cause is not addressed.”
Q. Will algae go away on its own?
Aqueon says brown diatoms in a new tank usually die off once it has cycled. Dr. Sanders, writing about algae that keeps coming back after cleaning, says: “Unless your tank is pea soup green or your fish cannot swim through, do not worry about it.”
Q. Will algae in my tank kill my fish?
The sources differ by type. Dr. Sanders calls algae “a sign that your tank is healthy”, while Aqueon says blue-green algae, left unchecked, “can suffocate live plants and even cause harm to fish”. If fish look unwell, test the water and contact an aquatic veterinarian.
Q. Does Dawn dish soap kill algae in a fish tank?
Keep it away from the tank. Dr. Sanders says “Do NOT use any detergent!” on decor, and Fluval’s canister manual says “NEVER use soap or detergents as traces of cleaning products could harm fish.”
Q. Can I clean algae with vinegar?
None of the sources here uses vinegar on algae. Aqueon’s checklist uses it only for mineral scale: “If there is a lot of calcium scale on the magnet, soak it in vinegar for an hour or two before rinsing and replacing.”
Q. Do LED lights cause algae?
Too much light can. Buce Plant says “If your lighting is too bright, or stays on for too long, this can cause algae to grow”, and Aquarium Co-Op says a planted-tank LED at 100% brightness “may be too strong and cause algae to flourish”, depending on the plants and the tank’s height.
Sources (19)
- Aqueon, Prevention and Control of Nuisance Algae (maker)
- Aqueon, Aquarium Cleaning Checklist (maker)
- Aqueon, Aquarium Do’s and Don’ts (maker)
- Aqueon, How To Fix Cloudy Water in a New Fish Tank (maker)
- API, Algae Scraper for Glass Aquariums (maker)
- API, AlgaeFix Algae Control (maker)
- API, Phosphate Test Kit (maker)
- Continuum Aquatics, AquaBlade (maker)
- Mag-Float, Glass products (maker)
- Fluval, AquaClear power filter manual (maker)
- Fluval, 07 Series canister filter manual (maker)
- Hydor, Theo heater manual (maker)
- Hydor, Do I need to turn the heater off when I am doing a water change? (maker)
- Dr. Jessie Sanders, DVM, Aquatic Veterinary Services, How to Clean a Fish Tank the RIGHT Way (veterinarian)
- TFH Magazine, The Skeptical Fishkeeper: Algae, Part 2 (magazine)
- Aquarium Co-Op, Top 5 Ways to Clean Algae from Your Fish Tank (retailer blog)
- Aquarium Co-Op, How to Balance Aquarium Lighting (retailer blog)
- LiveAquaria, Easy Ways to Control Algae Growth in Aquariums (retailer blog)
- Buce Plant, Aquascaper’s Guide to an Algae-Free Tank (retailer blog)
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