How To Get Rid Of A Bacterial Bloom In An Aquarium?

Experienced aquarium owners are well aware of the range of issues that can affect our tanks, from out of control algae, through to parasites and invasive species.

Aquariums are a delicate ecosystem and the balance that keeps everything alive can be easily knocked out of control.

Sometimes an issue is very easy to fix, just requiring time to naturally rectify the issue. At other times we’re required to carry out multiple water changes, quarantine fish or dose the aquarium with chemicals.

A bacterial bloom is one such issue, and it most commonly found in new tanks, but I can affect well-established aquariums if the conditions are right.

What Is Bacterial Bloom & How Do You Identify It?

A Bacterial Bloom which is also referred to as “new tank syndrome”, due to the fact many new aquariums will go through this phase.

It will often look like white, cloudy or milky water. The colorization is actually made up of millions of aerobic bacteria suspended in the water.

As the nitrogen cycle in an aquarium begins, nitrate and phosphate levels in the tank will begin to fluctuate and rise. The increased levels of nutrients will facilitate the rapid spread of bacteria throughout the tank.

Contrary to popular belief, its the increase in bacteria in a tank which causes a spike in ammonia levels, and not the other way round.

The number of bacteria present can be so intense that you might struggle to see your fish.

What Causes A Bacterial Bloom

There are a number of causes of a bacterial bloom.

New Aquarium

In a new aquarium, a bacterial bloom will tend to occur after a couple of days once the tank has been set up. This is part of the normal tank cycle and should clear up after a couple more days as equilibrium is found.

It happens because the water chlorine levels will drop during the first few days. This drop will allow any bacteria present to flourish and begin the process of consuming any biological materials present.

A second bacteria bloom can occur once fish are added to a tank as more biological material will be present.

It’s for this reason and others that I would suggest only adding a few fish to your aquarium at a time. You need to allow your tank to balance itself out and for beneficial bacteria to establish themselves in the tank.

If the bloom fails to dissipate after a couple of days, I would suggest manually adding beneficial bacteria.

Established Aquarium

A spike in the available nutrients can cause a bacterial bloom to occur in established aquariums. There are a number of reasons for a spike in nutrient levels:

  • Dead fish not being removed from the tank, allowing them to decompose and release chemicals into the water.
  • Dead plant matter not being removed from the tank, causing nutrients to be released into the water.
  • Significant disturbance to the substrate can cause locked away nutrients and bacteria to be released.
  • Equipment failures such as filters running not working or being overworked.
  • Overfeeding of fish will produce an excess of nitrates and phosphates.
  • Adding a large amount of fish to a tank at a time can cause a bloom before the aquarium is able to create a natural balance.

Can A Bacteria Bloom Harm Your Fish?

A bacterial bloom will not harm your fish directly, the bacteria are pretty harmless, but reduced oxygen levels can cause stress to your fish.

As bacteria are aerobic in nature they will consume oxygen in the tank… and a lot of it.

You may notice your fish gasping near the surface during a bacterial bloom. This is a result of bacteria processing all of the available oxygen in the tank. While it’s unlikely that your aquarium inhabitants will suffocate, you can reduce the stress levels by increasing oxygenation of the water.

The best way to increase oxygen levels is by agitating the surface of your aquarium. If you have a filter pump with a nozzle, angle the nozzle so it points towards the surface of your tank. Alternatively, physically move the pump so that the outlet agitates the waters surface.

You can also introduce an airstone in the tank which will further agitate the waters surface.

How to Get Rid Of a Bacterial Bloom in an Aquarium

Every aquarium can expect to encounter a bacterial bloom at some point, even if its just during the initial tank set up. While most will disappear after a few days, there may be times that you have to manually intervene.

So, how do you get rid of a bacterial bloom in an aquarium? For a quick clear up you can use an enzyme such as Nutrafin’s Bio-Clear. Simply dose the tank according to the instructions and you’ll quickly find the bacterial bloom disappearing.

For additional methods and preventative methods, read on 🙂

Inspect and Fix Your Aquarium Filter

If your aquarium filter is broken, blocked, dirty or malfunctioning in some other way. Fixing it can remove a bacterial bloom overnight.

You need to make sure water is being pumped through the filter medium at sufficient volumes and that your filter medium is not clogged up with slime and dirt.

If water leaves your pump at a trickle, then there’s an issue somewhere. A quick check of the fan blades and pipes may indicate a problem, just make sure you turn it off at the wall before doing anything.

If there’s an issue which you can’t diagnose then I personally wouldn’t like to take the pump apart. These things are meant to be submerged in water with mains power going into them, any amateur repair jobs just sound like a recipe for disaster.

Check out our best aquarium filter article for some suggestions of a replacement.

Perform a Water Change

Water changes should be part of your regular tank maintenance. But, if it’s been a while since you’ve performed one, then its maybe time to carry one out.

If the bloom is particularly bad then a 30% water change might be advisable. The water change will help remove excess nutrients, just make sure you vacuum your substrate at the same time to remove as much organic material as possible.

Feed Your Fish Less

Excess food can cause bacteria to flourish.

If this is you, then stop feeding your fish so much. All food should be eaten in roughly two minutes, if there’s still food hanging around after 5, then you’re probably feeding too much.

Once you’ve stopped feeding so much food, then try and clear up some of the excess. Old food and waste has probably permeated your substrate, so use a substrate vacuum to suck up as much as you can.

A couple of days after making these changes you should find the bloom clearing up.

Introduce Beneficial Bacteria

Another excellent way of eliminating a bacterial bloom is by introducing beneficial bacterial. We can recommend a couple of products, both Seachem’s Stability or Nutrafin’s Cycle have worked for us.

Both of these products work to reduce the nitrogen cycle by introducing a ton of good bacteria. I use Seachem’s everytime I complete a water change.

I would suggest dosing your tank a couple of times a day for three days straight. This should clear up a bloom in no time.

Introduce Enzyme’s

An Enzyme mixer such as the Nutrafin’s Bio-Clear helps in speeding up the processing of nutrients, drastically reducing the time a bloom is present for.

You can either choose to regularly dose your tank, which well help stabilize your tank conditions and process waste materials.

Alternatively, you can use enzyme’s if you perform any major changes to your tank which could cause a bloom, or when a bloom has started.

How To Prevent A Bacterial Bloom

Prevention is of course better than a cure. I would much rather put a little effort into preventing something rather than a ton of effort into fixing it.

The methods of preventing a bacterial bloom are actually pretty similar to the cures, it’s largely a case of proper maintenance. But to summarise the key points:

  • Make sure your aquarium filter is in good working order
  • Clear up any dead plants, fish or uneaten food
  • Perform regular water changes and clean the substrate while you’re at it
  • Dose your tank with enzyme’s and beneficial bacteria if you need to
  • Monitor your nitrate and phosphate levels and take action before a bloom occurs

Final Thoughts

A bacterial bloom is rarely a major issue for an aquarium. There are so many ways to tackle one that you should be able to find one that works for you.

If you have experience with any of the methods discussed above, or have information to share that we’ve not covered above, we’d love to hear from you. Please let us know in the comments below.


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