Cloudy water in new fish tank.

How to Fix Cloudy Aquarium Water: Diagnostics and Solutions

New fish tanks are like freshly planted flowers — you cannot expect them to be beautiful and to smell nice. In fact, quite the opposite is true in the early weeks. They are susceptible to all sorts of messes, life finds it hard to establish, and they may look (and smell) funny.

Cloudy water is among the most common sights in this period. It is commonly caused by four things:

  1. Dirty substrate
  2. Bacteria bloom
  3. Overfeeding
  4. Overstocking

How to fix the cloudy water in your aquarium quickly?

It is important to identify the main contributor to water cloudy in order to apply an adequate solution.

Cloudy water can be cleared through water changes, more powerful filtration, reducing the amount of food, and thinning the population of your tank. As far as dirty substrate or bacteria growth are concerned, a more passive but equally effective approach to clearing the water column is patience.

You can tackle each of the four major culprits for cloudy water in different ways. In most cases, regular water changes will help, but they might not be enough to resolve the issue in the long run.

It’s important to understand that cloudy water isn’t that big of an issue. In the early days, it is almost guaranteed to appear for a while. If it shows up in an established tank and is addressed promptly, nothing bad will happen to your fish.

Let’s examine each cloudy water cause in detail and understand how to deal with them.

1. Dirty Substrate — Wash Before Inserting

Dirty substrate is the most common reason for a cloudy water column. Whether it’s bought from the local fish store or from the hardware store, inert substrates like sand or gravel must be washed thoroughly.

On average, substrates from the fish store are cleaner, but they must be rinsed as well. Placing them directly inside your tank is very likely to result in cloudy water for weeks. Before putting them in, wash them well with running water. You can do that by putting the substrate in a bucket or through a sift. I combine the two methods.

First, I sift the sand or gravel under running water, then give it a good rinse in a bucket, to remove larger floating particles that the sift catches.

Now, if you’ve placed unwashed substrate in your tank and the water is cloudy, the quickest way to purify the water column is to take the substrate out and wash it. If you’ve planted plants already, use your hand to stir the bottom as much as possible without disturbing the vegetation.

This way a lot of the dirt will rise into the water column. Siphon it away in a large water change. If you are yet to place livestock, don’t be afraid to remove 50% or even 60% of the water column.

Repeat these aggressive water changes over the next few days and things should start becoming clearer soon.

A powerful filter can help here as well, but this is a temporary issue so investing in more expensive equipment might be unjustified.

Keep in mind that even the most thoroughly washed new substrate is likely to cause some cloudiness in the first few days.

2. Bacterial Bloom — A Natural Development

The aquarium hobby is fascinating. What we are trying to achieve is to capture a slice of nature in a box. And nature is incredibly complex. Filters, substrates, plants, water chemistry, and fish form the visible and measurable part of this enclosed biodome.

But thousands of invisible biochemical processes are taking place all the time. The vast majority of them are performed by countless bacteria that essentially transform matter. They are crucial for the survival of your fish tank; they are the most active contributors to the nitrogen cycle.

The specific bacteria species are numerous, but when it comes to cloudy water, there are two major populations:

Heterotrophic Bacteria

These are bacteria that rely on external food sources to survive and multiply. In fish tanks, heterotrophic bacteria consume the uneaten fish food, poop, and decaying (plant) matter.

The end result of their activity is ammonia but also more bacteria. Much more bacteria.

They reproduce in, quite literally, minutes. It is these heterotrophs that, if left unchecked, make the water cloudy. Usually, their population explodes in newly-set tanks, but bacteria blooms happen in established aquariums too. The reason why they appear in new tanks is the presence of organic matter and the lack of animals that eat them. They form a biofilm we see as cloudy water. 

Autotrophic Bacteria

Autotrophs eat inorganic matter, like ammonia, transforming it into less harmful substances. They are essential for the nitrogen cycle. One way to think of it is that when colonies of autotrophic bacteria are established, the cycle is complete.

The thing is, autotrophic bacteria take considerably longer to reproduce (about 24 hours compared to 30 minutes or so for the heterotrophs) and need a place to call home.

Filter mediums like sponges are where most autotrophs reside.

How to Deal with Bacteria Blooms

Autotrophic bacteria help balance things out. Cloudy water in a new tank can be left unchecked for a few days. Chances are, this tactic will give time for autotrophs to grow in numbers and they will purify the water.

Alternatively, regular water changes in the first few days will remove some of the biofilm heterotrophic bacteria have created and will decimate their populations.

I prefer the former approach, as it is less labor intensive, but it also shows you when the cycling of the tanks is complete.

On average, regular water changes of 20% or so clear up the water a couple of days sooner, but letting nature take care of things helps “season” the fish tank better.

In case bacteria blooms appear in fish tanks that are several months old, keep on reading.

3. Overfeeding Your Fish — A Common Tendency

Too much food can result in cloudy water. It won’t happen overnight and there will be other signs that you are overfeeding your fish.

Overfeeding is a very common and understandable tendency among new fish keepers. For one, the default setting of the little swimmers is to beg for food every time you approach the tank. Couple this with the fact that feeding them is interesting to observe and that we want to keep our fish happy, and we end up with buggers that eat too much.

But overfeeding has side effects. It cannot kill fish directly, but it certainly will affect negatively the water chemistry. Seeing cloudy water is a strong indicator that things have gone too far. Food uneaten by your livestock fuels the proliferation of the heterotrophic bacteria discussed above.

What’s even worse, decaying food can produce deadly ammonia spikes.

The easiest and most direct solution is to reduce the amount of food and the feeding frequency. Fish can go several days without eating. If you have had your aquarium for a few months and its water becomes cloudy, stop feeding for two or three days and see whether that changes things. 

Excessive food can contribute to bacteria blooms, among other things.

Don’t be afraid to give them food every other day or, indeed, very small doses. Start with the tiniest amount of food possible and increase it until you find the dose your fish can eat completely in 2-3 minutes. That’s more than enough when you feed them once a day.

4. Overcrowded Tank — No Fun for Anybody

An overstocked tank can result in cloudy water, and it is a considerably worse situation than overfeeding or dirty gravel. The solution is obvious and simple — take some fish out. But where do you put them?

Giving them away to other fishkeepers is a quick way to make new friends and help your fish feel more comfortable. Overstocked aquariums, while common in many pet stores, stress the fish and diminish the quality and duration of their lives. Cloudy water is the least of your worries in such situations.

If no friendly fishkeepers are around, talk to the local fish store to exchange a few common fish for a slightly more expensive one that doesn’t mind living alone. Or trade them for live plants or food. Whatever bargain you can strike, it will help your fish feel better.

Lastly, if you don’t have a hospital tank handy and don’t want to invest in one — even though it is a great idea — a bucket or a large plastic box can be a temporary solution. Just make sure to fill it up with aquarium water and maintain the appropriate temperature.

Other than that, a more powerful filter will help with cloudy water. More fish produce more waste and, yes, this waste feeds bacteria blooms. Powerful filtration will take care of the excess waste, but reducing the amount of fish is infinitely better than reducing your bank account.

One caveat here is that too many fish in a new tank is not necessarily the same as in an established aquarium. New fish tanks lack the beneficial autotrophic bacteria as they take time to establish large colonies. So, a more mature tank can handle more fish without the water ending up cloudy. Of course, this is valid up to a point, as any tank can be overstocked.

Additionally, many starting tanks don’t have any or nearly enough plants.

The Crucial Role of Plants

Plants are active contributors to the water balance in your aquarium. They consume all sorts of things from the water column, actively filtering it, and balancing it 24/7. Their benefits stretch far beyond helping with cloudy water and are a worthy addition to any fish tank.

The issue is that live plants require a slightly more specialized setup, but the investment in time to understand how plants live and equipment is totally worth it. The difference in cost isn’t that great and can be spread over time, to make the purchases more manageable.

Crystal Clear Conclusion

Cloudy water is a rather natural occurrence, especially in new tanks. If it’s caused by a bacterial bloom, given enough time, the aquarium will regulate itself. However, it is important to recognize what may be making your water cloudy.

Overstocking the tank and feeding your fish too much will lead to more serious problems than suboptimal aesthetics. Understand the underlying reason and you will know how to fish cloudy aquarium water with minimal, strategically applied effort.

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