Wondering why your tank water is cloudy and how to fix it is a perennial problem that comes up often in our discussion forum. There are a variety of reasons why your saltwater aquarium water may be cloudy. In order to figure out what the problem is—which is the first step toward solving it—there are a few things to examine:


This image comes from the Reef2Reef archives.

1. How old is your aquarium? New tanks often have different problems than mature tanks.

2. Did it come on suddenly or gradually?

3. When did you first notice it and at what time of the day? There are typically swings in the chemistry of the water column between night and day.

4. Has anything changed recently in your aquarium? Have you added or taken away anything? Has anything died? Is anything sick?

5. When did you last do a water change, and how much did you change?

If you ask anyone for advice, in addition to the questions above, people will also need to know these things:

1. What is your tank size?

2. What kind of filtration do you use?

3. How much live rock and/or sand do you have?

4. What kind of sand do you have?

5. What are your tank chemistry parameters? i.e. pH, nitrate, nitrite, phosphate, temperature, alkalinity, and total dissolved solids (TDS), for example.

6. Do you dose anything like limewater?

7. How much flow does your aquarium have?

Here are the most common reasons why your tank is cloudy, and some quick ideas to solve the problem.

Precipitation: It’s possible that something, likely calcium carbonate, is precipitating in your aquarium water. Dr. Randy Holmes-Farley wrote a very detailed article about this topic which you can study at your leisure. And Dr. Holmes-Farley moderates a chemistry forum on reef2reef where you can ask questions to your heart’s content. In the briefest of terms, calcium might precipitate if there’s too much calcium and/or a spike in pH and/or a lack of magnesium, and all of these things are temperature related—that’s why calcium often deposits on and coats your in-tank heater. Please note that if you have a blizzard of precipitation, any calcium or alkalinity tests will not be accurate.

Solution: As in most things in reefkeeping, there’s not a one-size-fits-all answer, and this is a complex topic. It depends on your individual set up. Did you accidently dose too much limewater? Then do nothing. Is your pH too high? Then try to bring it down. You have to look at all the details and consider the big picture. And if you can’t figure it out, then by all means post your question to the forum.

Sand: Have you recently added sand to an aquarium filled with water? Have you recently added a critter (or several) that stirs up the sand bed? Do you have powerheads unintentionally pointing down at the sand bed? A sandstorm can definitely create a cloudy tank.

Solution: Usually the answer is to do nothing. Some people run a several micron filter to clear it up quickly, but if you adjust your powerheads and flow in the right directions, sand will settle on its own.

Microbubbles: Microbubbles in the aquarium can be challenging to solve. Some possible culprits include a skimmer that isn’t adjusted properly, too much turbulence or cavitation in water in pipes headed down from the display tank to the sump or up from the sump to the display tank, not enough time for bubbles to dissipate when water flies through the sump, and a leak in plumbing that is drawing air in where it’s not supposed to. Obviously, some bubbles are good because they help with gas and oxygen exchange. Microbubbles per se are not bad for your livestock—think of crashing waves on Oahu’s north shore—but if you have so many bubbles that your tank looks like white static on a television screen, then that may not be the crystal clear look you want.

Solution: Figure out the source of the problem. Some people slow the flow through the sump, decrease tank turnover, and instead add circulating pumps inside the display tank to increase flow. Others might add over and under baffles into a sump to give bubbles time to break. Take a close look at your skimmer to see if it’s adjusted properly. Maybe the skimmer needs to sit closer to where the water arrives into the sump so bubbles can break before leaving the sump via a pump. Check your plumbing for tiny leaks and examine closely the water arriving into the sump and arriving into the display tank to see where the problem lies. Adjust as needed.

Bacterial or Algae Bloom: If your tank is new or you have recently added new critters, this is the mostly likely cause of cloudy water. You need bacteria in your aquarium. Bacteria is what converts ammonia to nitrites to nitrates, which is what you want. In other words, the bacteria takes compounds that are harmful to your living creatures and converts them into less harmful compounds. This is also called a “cycle” or “cycling” the aquarium. As you add more fish or invertebrates, or you feed more, or something dies, waste is created, and the bacteria have to increase to cope with the higher bio-load. A fast increase in bacteria can cloud the water.

And although all new aquariums go through a cycle, the cycle is not a one-time thing. Your reef aquarium will go through several cycles throughout its life large and small whenever there are significant changes in the bio-load. Some of these cycles will be obvious. Some will not.

Cloudy water could also be a bloom in algae. If the cloudiness is from microorganisms, and it lasts a long time, some people will add a UV filter, at least temporarily to help clear the water. The good bacteria that you want and need will likely (eventually) colonize your rock and sand and not be free-floating, so adding a UV filter will not destroy your biological filtration. But it can help clear the water.

Solution: Usually the solution is to do nothing and wait, especially if the tank is very young. Sometimes a UV filter will help.

Spawning: Spawning is last in our list because it’s probably the least common of the four most common causes of cloudiness. But it does happen, and it can make your tank white like you were circulating full-fat milk. Sometimes spawning is because you have a super healthy tank, and for example, several anemones could spawn at the same time. See Kevin Kuykendall’s article about that in Reefkeeping Magazine. In other cases, it’s possible for livestock to spawn because it’s unhealthy—a kiss before dying, so to speak. A tank turning completely chalk white with spawn is an emergency.

Solution: The only thing you can do is start doing water changes and remove any dead livestock as soon as possible because decomposing critters make the water chemistry problem worse. If you didn’t notice what was happening right away, you may have some losses. If there’s a small spawn from a coral, you may not have to do anything. If your fish spawn, maybe you’ll want to try to collect some of the eggs to raise (good luck with that). Some people try to add oxygen, which has its own set of problems. Some might try running carbon. If you’re not sure what to do, we even have a Tank Emergency forum.

When you have a problem with your reef tank, try to stay calm. Look at everything that is going on in your tank, everything that has happened recently, and then you can have a much better idea how to proceed. A cloudy aquarium is not unusual and not necessarily a big problem. In fact, it may be entirely normal, and this too shall pass.

Because of the complex chemistry of a reef aquarium, everything you choose to do or not do will have an effect on something else, for example adding a fish or removing some algae. Every time you dose something or filter something else out, that affects the whole system. Running carbon for example, can be a dream come true: it makes murky water clear and can remove a lot of organics. But granular activated carbon called GAC has an unpleasant side effect too—it can cause HLLE disease in fish.

This is why it’s so important to look at the big picture and move slowly. Find the way to best address your specific tank’s problem while least damaging the inhabitants. The topics briefly covered today are actually vast topics that could each be discussed at great length. We hope this quick cloudy water guide has been helpful to you.


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Author Profile: Cynthia White


Cynthia received her BA in English from NYU in 1492. She has been a freelance writer and editor for over 20 years. She has written articles for newspapers and magazines, both in print and online, as well as ebooks, press releases, and sales and marketing copy. She was formerly a Marketing Manager for a small oil company. Her portfolio can be found here. Now she is a writer and editor on staff at R2R, where her forum nickname is Seawitch. Her build thread can be found here.

For 15 years, she kept a dozen freshwater tanks, bred cichlids--Cyphotilapia frontosa--and sold them to pet stores in Calgary. Finally, after years of study and hand-wringing, she has come to saltwater side. She lives in British Columbia, Canada, with her husband and three special-needs dogs.