The short answer
Prevent what clouds it. Cycle a new tank first, size the filter to your tank, stock lightly, feed only what fish eat in under 2 minutes (Aqueon) and limit light hours. Change part of the water on a schedule (Aqueon: weekly or every two weeks) while vacuuming the gravel, and rinse filter media in tank water.
Freshwater steps. For water that is already hazy, see why a fish tank turns cloudy.
What you need
Cleaning tools and supplies:
- A gravel vacuum and a bucket kept for the tank. Aqueon, an aquarium equipment maker: “Don’t use buckets, scrub pads or any other equipment on your aquarium that have been used with soap or other chemicals.”
- A conditioner that “treats chlorine AND chloramine” (Dr. Jessie Sanders, an aquatic veterinarian), a thermometer and a light timer.
- A test kit. Aqueon: “Testing is the only way to know if your aquarium water is safe.”
The kit below is compared in our freshwater test kit roundup, which notes it leaves out GH and KH.
| # | Preview | Product | Price | Link |
|---|---|---|---|---|
| 1 |  | API Freshwater Master Test Kit 800-Test Freshwater Aquarium…Liquid master kit | $35.98 Prime | Check on Amazon |
The routine, step by step
Cycle a new tank
Aqueon lists “Not cycling the aquarium” among beginners’ common problems and calls adding “just a few fish initially”, feeding sparingly and testing “the safest way”. Our cycling guide covers both methods, and our beneficial bacteria roundup compares starters. While a tank is new, Aqueon advises against “vacuuming the gravel or cleaning the filter when it doesn’t really need it, as this can destroy beneficial bacteria”. It also says regular partial water changes are the “#1 thing” “except during the first 30 days.”
Size the filter to your tank
Aqueon’s problem list: “For best results, purchase a filter that is rated for an aquarium a little larger than the one you own.” Its filter guide: “recommended for your specific tank’s volume so you don’t under-filter or over-filter the water”. Fit media during steps 6-10, unplugged. Compare models in our filter roundup.
Stock lightly
Aqueon: “Don’t overstock your aquarium. Fish that are crowded may fight, and overstocking can lead to water quality problems.”
Feed small amounts
Aqueon: “Don’t feed more than your fish can consume in under 2 minutes. Uneaten food can pollute the water and stress your fish.” Unplug first (step 6) for leftovers; plug back in after (step 10).
Limit light hours
Aqueon’s algae page: “A maximum of 6 to 8 hours of light is sufficient in unplanted aquariums, while planted aquariums need 10 to 12 hours of high-quality light per day.” Its cloudy-water guide differs (“may need eight to 12 hours” planted; “can handle six hours or less” unplanted). See our guides to green water and algae.
Before your hands go in, unplug everything
Fluval’s 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, an aquarium equipment maker, gives two heater waits: “at least 5 minutes before taking it out of the water or before adding or changing the water” (Theo manual) and “at least 30 minutes before any waterchange” (support page). Follow your heater’s manual; with none, our reading is the longer wait, at least 30 minutes, before water comes out. Unplug the same way before any hands-in maintenance task.
Test the water
Sample while still unplugged. Aqueon: “Use an aquarium test kit once a week to test your ammonia and nitrite levels, which ideally will be close to zero.” API: test “weekly-biweekly”. For a reading above zero, see how to lower ammonia or nitrite.
Vacuum the gravel while you take out part of the water
Still unplugged. Dr. Sanders: “Press the end of the siphon into the substrate, allow the gravel or sand to move 1/4 to 1/3 of the way up the tube, then pull the siphon up, breaking the siphon.” In a new tank, see step 1 (also for step 9). Amounts: table below; full steps in our water change guide.
Rinse filter media in the water you took out
Still unplugged. Dr. Sanders: “Do NOT use tap water. The chlorine in tap water that makes it safe for us to drink kills your good bacteria.” Fluval’s 07 canister manual: “Never replace all biological media at one time, as valuable colonies of beneficial bacteria would be lost.” Aqueon’s cleaning checklist: “Replace any chemical media such as activated carbon.” See our filter cleaning guide, and cleaning a fish tank.
Refill with treated water, then plug everything back in
Aqueon: “Replacement water should be the same temperature as the aquarium water.” and “Always treat tap water with a water conditioner to neutralize chlorine and ammonia before adding it to the aquarium.” Hydor: “The Heater can ONLY then be turned back on when the Aquarium is full and the heater is completely submerged.” Fluval, for its AquaClear power filter: “First, ensure the filter case is completely filled with water” and “Do not allow filter pump to run dry.” Its 07 canister manual: “never plug the filter in unless the canister is completely full of water.” Other filters: follow your manual. Evolution Aqua, for its UV unit: “Fill the system with water before connecting the appliance to the power supply.” and “Do not operate the appliance without water flow.” After a quick job with no water removed and the filter unopened, plug back in once hands are out (our wording).
Water change percentage and frequency, by source:
| Source (type) | Routine amount | Most in one change |
|---|---|---|
| Aqueon (maker) | 10% weekly or 25% every two weeks | “Avoid water changes of more than 25%” |
| API (maker) | 10-30% if levels are elevated | “not recommended to do more than 30%” |
| Dr. Sanders (aquatic vet) | None on this page | “Do not remove more than 50%” |
“Most in one change” is a ceiling, not a target; the sources disagree. The reasons: Aqueon, “larger water changes can stress your fish and upset the balance in your aquarium”; API, “it may cause loss of bacteria and be stressful on the fish”; Dr. Sanders, “to avoid sudden swings in temperature and pH.”
Carbon, clarifiers and UV: optional extras
- Activated carbon. API says its carbon “clears aquarium water by removing organic waste, colors, odors, and dissolved medications”; “Replace every month or sooner if aquarium is heavily stocked and/or colors and odors are detected.” Its page also warns: “WARNING: This product can expose you to chemicals including Carbon black, which is known to the State of California to cause cancer.” Fit carbon as in step 2. More in why use carbon in a filter.
- Clarifiers. API says Accu-Clear “combines tiny particles together to form larger clumps so they can settle at the bottom or be removed by your aquarium filter”. Its label: “Do not overdose.” In saltwater, it “will not clear cloudiness and could even make it worse.” API’s FAQ adds that “it does not remove harmful chemicals, such as ammonia and nitrite, from your aquarium that may be an underlying cause of cloudiness and poor conditions.”
- UV sterilizers. Aqueon: “Install a UV sterilizer to clear up green water in a few days.” Not for a milky bloom: see Mistakes below. Evolution Aqua, a UK filtration maker, in its evoUV manual: “WARNING: Ultraviolet radiation. Avoid exposure to direct or reflected radiation.” “Do not operate the appliance if the protective enclosure or protective parts are removed or damaged.” “Always disconnect the appliance from the power supply before carrying out maintenance, cleaning or lamp replacement.”
Mistakes and troubleshooting
- Over-cleaning. Aqueon: “Don’t completely tear down your aquarium to clean it. This will destroy the biological balance with disastrous results.”
- A milky bloom. Aquarium Co-Op, an aquarium retailer: “Don’t add a UV sterilizer or do lots of water changes to remove the haziness; this just makes the bacterial bloom last even longer.” Its fix: “wait one to two weeks, and the water will gradually clear up on its own”.
- Fish look unwell. Test the water and contact an aquatic veterinarian.
Ready to buy?
Our water clarifier roundup compares nine products. Water Care & Testing has test kits and conditioners; Tank Setup & Care, more routine guides.
Frequently Asked Questions
Q. What can I put in my fish tank to keep the water clear?
Carbon, a clarifier or a UV unit (limits above; milky bloom: see Mistakes).
Q. Can I keep the water clear without water changes?
API: “Yes, it is very important to perform partial water exchanges”, even with a large filter.
Q. What is the fastest way to clear cloudy water?
For cloudiness “caused by floating particles”, API says Accu-Clear water “will clear within several hours once it is dosed”; it does not remove ammonia or nitrite. For a milky bloom, see Mistakes below.
Q. Why is my fish tank water getting dirty so fast?
Aqueon, on new tanks: “If your aquarium or filter seems to need cleaning in the first 2 to 3 weeks after setup, the filter may be too small for the job, you may have too many fish, or you may be overfeeding.”
Sources (17)
- Aqueon, Do’s and Don’ts (maker)
- Aqueon, 15 Common Problems (maker)
- Aqueon, cloudy water guide (maker)
- Aqueon, nuisance algae (maker)
- Aqueon, filter guide (maker)
- Aqueon, Cleaning Checklist (maker)
- API, Routine Aquarium Care (maker)
- API, FAQs (maker)
- API, Super Activated Carbon (maker)
- API, Accu-Clear (maker)
- Fluval, AquaClear manual (maker)
- Fluval, 07 manual (maker)
- Hydor, Theo manual (maker)
- Hydor, support page (maker)
- Evolution Aqua, evoUV manual (maker)
- Dr. Jessie Sanders, cleaning a tank (aquatic vet)
- Aquarium Co-Op, cloudy tank (retailer)
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