Why Is My Goldfish Tank Cloudy?

You spent ages carefully setting up your goldfish tank, and you give your fish the best care, so why is your goldfish tank cloudy?

That dreaded murky aquarium water could be down to a bacterial bloom if the tank hasn’t cycled properly yet, you might have forgotten to wash dusty gravel, or there could be a more sinister reason. 

Read this guide to find out why your goldfish tank water has turned cloudy and, most importantly, how to fix the problem!

What causes water in a fish tank to turn cloudy?

Before you start trying to fix the problem of cloudy water, you need to know what’s causing it.

First of all, what colour is the water? 

There are different shades of cloudy, and the colour of the water tells you what’s causing the problem, as well as giving you clues as to how you can fix it.

White or Grey Tank Water

There are few things that can cause grey or white water in fish tanks.

Gravel Residue

If you have a new goldfish tank and the water turns a cloudy white or grey when you fill it with water for the first time, the problem is almost certainly caused by dirty, dusty residue coming off the gravel substrate.

The Solution

This problem has a very easy fix! 

All you need to do is empty the tank, remove all the gravel, and rinse it thoroughly under running water until the water runs away clear.

If you don’t want to empty the tank, you can use water additives known as flocculates to help clear the water. Flocculates cause minute particles of floating matter to stick together so that the mechanical filter element of your system can remove them from the water. 

In your fish store, flocculates are usually labelled as “water clarifiers.”

Dissolved Constituents

If you cleaned the substrate, but the water is still cloudy, the problem is likely caused by high levels of silicates, heavy metals, phosphates, or other dissolved constituents.

You can confirm that by testing your tank water using an aquarium water testing kit. If the water is very alkaline with a high pH level, that could be the problem.

The Solution

Try dosing the water with a pH buffer or tap water conditioner. 

Alternatively, if the tap water in your area is a real problem, you could use Reverse Osmosis (RO) water instead.

Most good fish stores sell Reverse Osmosis water, although that can work out quite expensive if you have a large tank. However, you can buy home RO units, which are much more cost-effective and are relatively straightforward to run.

Bacterial Bloom

If your goldfish tank is newly set up and your water turns milky-white after a couple of days or a week, the phenomenon is probably due to bacterial bloom.

Bacterial bloom happens when a new biological filtration system is still maturing. 

Don’t turn off your filtration system!

You can accidentally trigger bacterial bloom by turning off your filtration system overnight. Don’t do that! 

You need to have your filter running 24/7/365. Only turn the system off briefly when you’re carrying out routine maintenance, such as changing the filter media or performing partial water changes.

Beware big water changes!

Changing too much water in your goldfish tank can accidentally trigger bacterial bloom, which can upset the aquarium’s eco-balance.

Removing too much water kickstarts a mini-cycle, causing bacteria colonies in the tank and filter media to repopulate. You can stay safe by changing around 20% to 25% of the tank water each week. If you need to change more water than that, always add a filter-boosting product to the water.

How long does it take for cloudy aquarium water to clear?

The problem usually fixes itself within a couple of weeks or months once the colonies of bacteria have become established and can cope with the amount of organic waste passing through the filter media.

You can give the beneficial bacteria a helping hand by using an aquarium vacuum to remove leftover fish food, dead plant leaves, and fish waste at least once a week. 

Reducing the amount of food you give your fish can also help to relieve the burden on your filter system.

Green Water

If you have a mature, established goldfish tank, cloudy green water is almost certainly caused by an overgrowth of algae in the water.

There are many species of algae that can grow in fish tanks. You’ll find algae growing on your plants, on decorations, on the glass viewing panes, and even on the gravel substrate! That’s not all bad since goldfish enjoy grazing on some nutritious algae species as part of their diet.

However, there are some forms of algae that live in the water, where they float around, giving the whole tank an unpleasant greenish tinge.

The Solution

Algae can be a real nuisance and a pain to get rid of once it’s established.

To remove floating green algae, you need to adopt a multi-pronged strategy, depending on what’s causing the problem.

Reduce Light

Algae love light! 

If you have your tank in a place where it’s exposed to sunlight, that will encourage algae to grow and spread throughout your goldfish tank.

To restrict algae growth, move your tank well away from sunlight. If that’s not feasible, screen the tank with drapes or blinds to block out the sun. You might also need to cut down the hours your fish tank lights are turned on, perhaps by fitting a timer.

If the algae are starved of the light they need, they will gradually die, and your filter system will get rid of the green algae from the tank water, leaving it crystal clear again.

Note: It’s not a good idea to place your goldfish tank in a spot where the sun shines on it! Sunlight shining through the glass creates hotspots in the tank and raises the water temperature. Goldfish are a cold water species; very warm water can kill them!


As well as light, algae need nutrients to flourish. 

Nutrients, such as phosphates, are derived from decomposing fish waste, fish food, and rotting plants. Those substances are effectively a fertiliser for the algae.

Starve the algae by cleaning your tank to remove phosphates from the water. Vacuum the substrate and trim away dead plant leaves and stems.

Now, test your tap water for phosphates. If the water is high in phosphates, you might need to switch to using Reverse Osmosis water for your goldfish tank or use a chemical treatment to remove the phosphates before topping up your tank.


Nitrates are given off by decomposing organic waste in your goldfish tank. These substances act as fertilisers for the plants and algae growing in your fish tank.

The level of nitrates in the water typically correlates to the amount of waste in your tank. Ideally, you want the nitrate levels to be below 30ppm for your fish to remain healthy. If levels get higher than that, you might see an increase in algae numbers.

If you keep up with your weekly partial water changes and maintain your filter system correctly, nitrates shouldn’t cause a problem. Make sure that your filter system provides a GPH (Gallons Per Hour) flow rate of at least four times the volume of water in your tank, ideally more.

Yellowish/Brown Water

Although it’s rare for a goldfish tank water to turn brown or yellow, that occasionally happens.

Bogwood and Driftwood

Bogwood and driftwood make attractive fish tank decorations and work well in a goldfish setup. You can train certain plant species to grow on pieces of wood, and your goldfish pets will enjoy grazing on the colonies of algae and beneficial microbes that grow on the wood’s surfaces.

However, the wood contains tannins that leach out into the water, staining it the colour of cold tea and lowering the pH level slightly. 

Although a “blackwater” environment might look unsightly, it’s actually not detrimental to your goldfish, as long as the pH level doesn’t get too low. In fact, certain species of tropical fish, including some tetras, originate from blackwater areas.

The Solution

Blackwater is easily prevented by soaking any driftwood or bogwood for a few days in a bucket before you introduce the decorations to your goldfish tank. If you use an activated carbon filter, that will remove some of the discoloration, too.

Most good fish stores sell pre-treated wood that won’t stain your water, but it’s worth double-checking that’s the case before you buy.

How to prevent cloudy fish tank water

It’s pretty easy to prevent your goldfish tank water from becoming cloudy.

As well as the solutions outlined earlier in this guide, there are several golden rules that you must follow to keep your goldfish tank water crystal clear.

Avoid Overfeeding Your Fish

Cartoon of a goldfish eating bloodworms from a packet

If you overfeed your goldfish, you are encouraging a bacterial bloom.

How so?

Well, as they decompose in the water, uneaten food and fish waste produce nutrition for bacteria. Uneaten food particles floating in the water can also make the tank appear cloudy.

We recommend that you feed your goldfish two small feeds per day, offering your pets only what they will eat in a couple of minutes. 

Goldfish can suffer from swim bladder problems if they are overfed, so a fasting day once a week when the fish aren’t fed anything can be beneficial to their health and help keep the water clear.

Don’t Overcrowd Your Goldfish Tank!

red and sunset orange fish circling pool of blue water

Overstocking your tank can cause several problems. Too many goldfish crammed into a small area can cause stress, which in turn results in health issues and disease outbreaks.

Too many fish will overload the tank’s filtration system. The beneficial bacteria in your biological filter media won’t be able to handle the amount of waste produced, leading to a bacterial bloom and cloudy water.

Add Activated Carbon To The Filtration System

If you add activated carbon to your filter system, that can remove tannins produced by wooden decorations that might stain your water a yellowish brown colour.

“Seed” The Fish Tank

If you’re setting up a goldfish tank from scratch, you can often prevent bacterial bloom and cloudy water by “seeding” the tank.

The process of seeding entails adding plants, substrate, and decorations from an established, healthy tank to your new one. Those items will carry a cargo of beneficial nitrifying bacteria that will jump-start the cycling process in your new tank. Seeding a new tank can cut the cycling time substantially and lessen the likelihood of the water becoming cloudy.

Final Thoughts

If the water in your formerly pristine goldfish tank suddenly goes cloudy, you’ll need to turn detective to determine the cause of the problem.

  • A new tank can become cloudy if it hasn’t fully cycled before introducing new goldfish, causing a bacterial bloom.
  • If your tank is exposed to sunlight or is lit for too long, floating algae might invade the water, turning it green.
  • Finally, if you have driftwood or bogwood in your aquarium to give it a natural look, tannins can leach from the wood into the water, creating a blackwater habitat where the water turns the colour of tea.

Did you cure your cloudy goldfish tank problem? Tell us how you did it in the comments box below!

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