Cloudy, hazy aquarium water is something every aquarist will encounter at some point or another. It makes for a terrible viewing experience and can put your aquarium inhabitants at risk. Turbid water will block light from reaching your corals and in the most severe conditions create a lack of available dissolved oxygen leading to the suffocation of fish and other aquatic animals.  

So how do I know what is causing cloudy water in my tank?  

There are two reasons aquarists encounter cloudy water, you either have particulates suspended in the water column or you're experiencing a bacteria bloom.   

Cloudy Water


You can be confident your cloudy water is caused by particulates if it occurs after adding new sand to your aquarium or after you have stirred up the sand bed during maintenance. 

In either case, it's just dust and detritus suspended in the water column which can be quickly removed with mechanical filtration. Change out your mechanical filtration every 12-24 hours until the cloud is gone.

If there is a particularly severe amount of very fine particulates in the water, it could clog up your fish's gills leading to suffocation. In these cases, employing a more aggressive "water polishing" approach with multistage mechanical filtration is necessary. 
Bacterial Bloom

If your aquarium was clear yesterday and you woke up today to find the tank hazy and cloudy, you are likely experiencing a bacterial bloom. This most often happens when cycling new aquariums while establishing a nitrogen cycle but can also plague existing aquariums. 

A bacteria bloom is a rapid reproduction of suspended heterotrophic bacteria that occurs because of a sudden increase in organic matter. 

Worst case, the bacteria can actually out-compete your fish and other aquatic pets for the available dissolved oxygen in the water leading to suffocation.

Seachem Clarity - Flocculant Additive

How To Clear Up Cloudy Water Caused By Suspended Particulates

When adding sand to a new aquarium, there is often some amount of cloudy water that occurs upon filling the tank. If using dry sand, you can negate this effect by rinsing the sand before using it.  With live sand, you can use a "clarifier" or flocculating agent to help speed up the process of removing particulates. Flocculants essentially bind smaller particles together creating larger particles that are easier to remove. 

In either case, you will want to ensure you have an effective form of mechanical filtration that is being cleaned and replaced every 12-24 hours until the tank is clear. Standard filter socks, filter pads, filter felt, and automatic filter rollers are going to be sufficient in most cases. 

Cloudy water caused by new sand

In severe cases or if you really want to clear the water in a hurry, you can set up a "water polishing" filter that runs alongside your standard mechanical filtration. A canister filter or internal filter filled only with sponges and mechanical filter pads is quite common but if you really want to step up the clarity game, you can use a sediment filter like what is used inside an RO/DI system. The Standard or Deluxe BRS Reactor Kit along with a brand-new sediment filter is what you need. Set it up to recirculate the water from your display directly through the sediment filter and you should achieve acceptable clarity within 12 hours. 

Preventing & Correcting a Bacterial Bloom

Bacterial blooms are sticky in that they can be persistent. In a new tank, it usually occurs through the process of establishing a nitrogen cycle when organic material is present and the bacteria go into a sort of reproductive hyperdrive. In existing tanks, you will see it happen if there is an unexpected death or if you disrupt a particularly dirty sandbed, both of which cause a sudden increase in organic material. Another common scenario is accidentally spilling a large amount of fish food into the water or overdosing on liquid foods; a sudden increase in organic material leads to the overproduction of bacteria. 

Bacterial Bloom

Regardless of the reason a bacteria bloom happens, the best remedy is time. You just have to wait it out and allow the bacteria to naturally balance out which will typically happen within 1-2 weeks. Ensure your protein skimmer is working and be sure your powerheads are creating surface agitation to maximize dissolved oxygen levels at all times.

While a water change might appear to help the problem short term, the bacteria bloom almost always comes back with a vengeance. 

In the case of a dead fish or tons of excess food, work to remove that excess organic material which will help to remedy the problem. If you let your sand bed get really dirty and then mixed things up causing a bloom, just leave the sand alone and give it time. Only clean small portions of your sand bed at one time; usually a 12" square area is all you want to clean at one time and then move on to the next area during your next water change the following week. This way your not stirring up too much waste at one time, reducing the likelihood of a bacterial bloom.