Bacterial Bloom In Reef Tank

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Today I woke up to a dead fish (Mr. Naso our Naso Tang) and a bacterial bloom in my 180-gallon reef tank and it really caught me off guard. I know the tank is fairly new since I upgraded from my 50-gallon cube to the 180 gallons just two short months ago.

I know that bacterial blooms are common and can happen especially in a new reef tank. There are several ways to combat this and I am going to share the best way to do this.

Before I do, I want to just say that a bacterial bloom happens when there is a need for bacteria to reproduce to clean up organic waste such as food, detritus, dead fish, or invert, etc. The bacteria reproduce much faster and float through the water column and sometimes more surface area is needed to house these bacteria.

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What Is A Bacterial Bloom

A bacterial bloom is a sudden increase of bacterial colonies in the water column of your reef tank or freshwater tank. When bacteria grow so fast it becomes visible to the naked eye causing cloudy water appearance in your reef tank.

A bacterial bloom can occur in an established reef tank or freshwater tank but typically only happens in a newly started aquarium due to the instability of the tank or the build-up of nutrients.

Heterotroph bacteria is usually the bacteria causing the problem as they don’t attach to surfaces as easily as their counterpart and are typically larger. They also reproduce much faster than autotrophic bacteria. Both bacteria can cause cloudy water but the most common are heterotrophs.

What Can Cause A Bacterial Bloom In A Reef Tank

Dead fish, invert and excessive feeding can cause this bacterial bloom to start. This is true for a freshwater and saltwater tank. Sometimes the water is extremely cloudy causing your tank to be an eyesore while other times the tank isn’t too cloudy but you can still see that it is.

If you have a dead or dying fish it is best to get it out as soon as possible. You could have a standby tank to house sick fish or inverts to skip the build-up of bacteria due to fish death but I get that it isn’t always possible to do this.

Who wants to flush a live fish that will likely die? It isn’t easy to do but if you want to skip excessive organic waste in your tank you should consider it. Even a 5-gallon bucket will work with an airstone for oxygen. This is only temporary to make the fish comfortable until it does die if you are uneasy with flushing a live fish.

Bacterial Bloom Not Going Away

I have had a bacterial bloom last over a week but never more than two weeks however I know people who have said their tanks had a bacterial bloom for 3 to 6 weeks.

If you have a bacterial bloom there are several things you MUST do right away. You need to consider any changes, fish deaths, broken equipment, or anything else that could have thrown off the balance and stability in your tank.

  1. Test Your Water – One thing for sure the water testing industry has leveled up over and over. We can test our water to see what our numbers are which can indicate possible issues to come or even diagnose issues. If you have excessive nitrate and phosphate you will need more bacteria to combat this which could cause a bacterial bloom to start.
  2. Check Your Reefing Equipment – If your skimmer fails or your bio pellet reactor isn’t working or getting water flow then this could cause an imbalance making your levels fluctuate which is not good depending on how fast and how much they fluctuate. If your skimmer isn’t working then there will be a build-up of organic waste.
  3. Consider Equipment Upgrades – If you run a skimmer rated for 100 gallons at a low bioload and you add maybe 5 more fish and have to feed heavier for higher energy fish like an Anthias then you may need to upgrade your skimmer and it is likely not doing the job it needs to do good enough. You could also consider upgrading your tank. If your reef tank isn’t large enough to house the livestock you have then your filtration may be working too hard for you.
  4. Check Your Equipment – Your maintenance habits could contribute to a bacterial bloom. If you have filter socks and don’t change them every few days they are a pointless mechanic in your filtration system. I change mine every two days (excessive I know) and I place the used ones in a bucket to clean when it is full. I clean mine once per month using a hose with a pressure nozzle that sprays hard on the filter socks to remove fish poop, food, and other organic waste. Be sure to turn them inside out and spray them well. I also soak them in bleach for a day and then let them dry out before being placed back into the sump.
  5. Time To Stability – One thing most new reef tank owners do is get in a hurry. This is bad in the reefing hobby. Adding fish slowly and letting bacteria build up and take up the surface area you have provided is the best thing for your reef tank. Provide a great place for bacteria to build up by using Brightwell Aquatics Bio Bricks, Bio Balls, and sand in your sump and reef tank.
  6. UV Sterilizer For The Win – A UV sterilizer will stop the bacteria in the water column from multiplying. This is not a permanent fix so be sure to keep good control of your nutrients and filtration.

How Long To Bacterial Blooms Last

A bacterial bloom typically lasts 3 to 5 days but can last up to a few weeks if the bioload is high in your reef tank.

My most recent bacterial bloom lasted just two short days and it was caused by starting a new tank with 9 fish and the filtration from the old tank. It was instantly cycled but still needed to stabilize.

If you experience a bacterial bloom for more than a week then it’s time to check your equipment and be sure that you have all of the proper equipment rated for your reef tank volume. Also, you need to consider any changes in feeding habits or other maintenance habits.

How To Clear A Cloudy Saltwater Tank

Bacterial bloom causing cloudy water to be present in this reef tank. Image Source: Jimbo from Reef2Reef

First, determine why it is cloudy. There are many reasons a saltwater tank could be cloudy. In order to clear it, you must determine the cause.

Here are a few questions to ask yourself:

  1. Did you disturb your sand bed?
  2. Did you just add sand?
  3. Are your powerheads blasting your sand bed?
  4. Did you just add fish?
  5. Did a fish or invert die?
  6. Did a fish, coral, or snail spawn?

If you recently added bacteria to your sand bed or added sand to your saltwater aquarium it is highly likely that this is the cause of your cloudy water.

If you just added sand to your aquarium then this is likely the cause. It could also be the sand bed being blasted by the powerheads moving water. Consider turning the power down some.

If you have recently added fish it is possible that this is a bacterial bloom where the bacteria in your saltwater tank are multiplying fast to make up for the nutrients in your tank.

Consider adding sand if you don’t have sand and more rock to your saltwater tank to add more surface area for the bacteria to grow on.

Do you really have a bacterial bloom? It is possible that you do not. Cloudy water could be from spawning activity in your aquarium. This does cause the water to cloud up and look like a bacterial bloom. If this is the case it will clear up in a day or two. Even if you think you have a bacterial bloom it’s best not to react for a day or two anyway just in case it clears up on its own.

A fish or invert dying could be the cause and you really don’t have much you can do sometimes. Especially when you have a large tank and don’t see the dead specimen.

Can I Add Fish During A Bacterial Bloom

It is not recommended to add fish or inverts during a bacterial bloom. You are only adding to the problem which may cause the bacterial bloom to last longer.

During a bacterial bloom, it is hard for fish to see through the water making them uncomfortable. I know my tangs and a few other fish find a rock and wedge between them due to stress.

To really answer this question consider how you would feel if you couldn’t see in front of you. When you are driving around with high fog it is very uncomfortable and stressful. You cannot wait to get through the fog.

Imagine fish feeling the same way. If you have to add fish due to an order that you are expecting before you had a chance to read this or before your bacterial bloom then check your ammonia levels as well as nitrate to ensure your environment is safe. Make sure this fish has the best chance of survival. Consider setting up a small 20-gallon tank for the new arrivals for the time being.

Will A Bacterial Bloom Kill My Fish

A bacterial bloom will not kill your fish but a lack of oxygen in the water could kill them. If you experience a bacterial bloom it is important to add more aeration to your water. I do this by adding another air stone to the sump providing more oxygen.

Most of the time your skimmer does a great job already but an added air stone can definitely help you sleep better knowing your fish aren’t being deprived of oxygen.

UV Sterilizer For Bacterial Bloom

A UV sterilizer will clear up a bacterial bloom usually within 24 hours if properly installed. A UV sterilizer will stop the reproduction of the bacteria cells allowing the water to clear up. This is not the solution to the problem though. You will still need to find the root cause of your bacterial bloom so that the UV sterilizer will help maintain but your tank will be stable.

What Size UV Sterilizer For Bacterial Bloom

Follow the manufacture’s recommendations on the UV wattage and purchase what you need for your tank otherwise you are wasting your money and the UV sterilizer will not fix your cloudy water.

Best UV Sterilizer For A Bacterial Bloom

I personally use the Coralife Turbo-Twist for my tank. It is not the best UV on the market but for the money and purpose, it is perfectly fine. I see a UV as something to maintain a problem and I want to find the solution so that a UV isn’t necessary just a temporary solution.

Coralife Turbo-Twist UV Sterilizer Black, 12X
  • Designed to remove unwanted free floating algae, parasites and bacteria
  • Increased exposure to UV light from twist-flow design
  • Mounting brackets included for hang on or in-line applications
  • Indicator light lets you know when the sterilizer is operating
  • Can be used with freshwater and saltwater aquariums

If you have a smaller tank then I suggest purchasing this UV here for 125 gallons or less in water volume.

Coralife Turbo Twist UV Sterilizer, 3X
  • Designed to remove unwanted free floating algae, parasites and bacteria
  • Increased exposure to UV light from twist-flow design
  • Mounting brackets included for hang on or in-line applications
  • Indicator light lets you know when the sterilizer is operating
  • Can be used with freshwater and saltwater aquariums

Bacterial Bloom After Water Change

If you have a cloudy reef tank after a water change then it is highly unlikely that the cloudy water is from a bacterial bloom.

It is possible that out of pure chance the bacterial bloom started after a water change but the likely cause is your substrate or sand being disturbed.

If you are experiencing cloudy water after a water change then allow the sand or substrate to settle and it will settle. Be sure to check your coral and rock for substrate and blow it off with a turkey baster.

How To Prevent A Bacterial Bloom

To help prevent bacterial blooms you can add live copepods which are part of the clean-up crew in your reef tank feeding on detritus and other organic waste. They serve other purposes too but do help as part of a clean-up crew.

Other clean-up crew members such as snails, sand sifting fish, feather dusters, crabs, etc will help prevent these blooms.

Another preventative method is your maintenance routine. I know I have already mentioned this but changing your filter socks, filter floss, and cleaning your skimmer will help prevent this. If everything is working properly and properly maintained the ounce of prevention is going to benefit your reef tank.

Consider upgrading your filter socks to a roller mat. This will keep all the waste buildup from decomposing in your sump preventing an excessive buildup of nutrients that keeps your bacteria from responding so harshly.

Bacterial Bloom In A New Tank And No Fish

If you have a bacterial bloom in a brand new tank without fish then it is likely going to pass in just a few days. This is likely caused by your cycle process and you probably added too much ammonia and caused a spike in nutrients.

Another cause of a cloudy aquarium that is newly set up without fish is the bacteria in a bottle you used. This can cause cloudy water after you pour it in for cycling your aquarium.

Did you disturb your sand? If you just poured the water in or did a water change you can guess it is not a bacterial bloom and it is just the sand floating through your water column.


A bacterial bloom can be somewhat serious but usually not. It is important to figure out what caused the bacterial bloom so that you can prevent it in the future. If you do nothing it will likely go away in just a few days but a UV sterilizer will work, but to keep the balance in your reef aquarium you will need to figure it out and fix the problem so that your fish and coral are happy and healthy.

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