One of the most common questions new aquarium owners ask has to do with cloudy or milky water. What does it mean? Why does it happen? And how can I fix it?

The good news is that cloudy water in a new fish tank isn’t necessarily an emergency situation. In fact, there are some very simple ways to diagnose and treat cloudy water and each will be discussed below.

Why Is My New Aquarium Water Cloudy?

Does your brand new fish tank suddenly have cloudy water after being clear for the first few days?

Don’t worry, a newly set up tank is a biological blank slate; there are virtually no life forms present on Day One. Within days, variety of microscopic organisms will begin trying to establish themselves in the tank.

The beneficial nitrifying bacteria that filter the water and create stability and balance haven’t had a chance to colonize the system yet, so it’s kind of a free-for-all for a week or so.

A variety of free-floating bacteria and other microbes take advantage of minerals and nutrients in the water and begin to multiply unchecked – thus causing the cloudiness.

The situation is sometimes compounded and exacerbated when hobbyists add too many fish all at once and/or feed too much, providing these microbes with an additional food source.

The Best Solutions for Cloudy Aquarium Water

A natural reaction is to “do something”. There is obviously something “wrong” with the tank, requiring action on our part.

However, taking measures to prevent and avoid cloudy water in advance is a far better approach than attempting to eliminate it once it starts.

When you begin to see cloudy water, it’s best to just let it run its course. 

Some common points to keep in mind to solve cloudy water.

1. Do not overfeed your fish.

Beginning aquarists often fear their fish will starve to death, so they feed heavily and often.

Unfortunately, there are few, if any, nitrifying bacteria present to break down the resulting waste or uneaten food, which the cloudy water bacteria take advantage of and continue to multiply.

Even worse, harmful ammonia and nitrite levels may begin to rise.

Fish in nature don’t always eat every day, and some predatory fish may only eat once or twice a week. No fish ever starved to death in three days.

2. Don’t put too many fish in your fish tank.

More fish mean more waste and more food for the microbes causing the cloudy water.

Too many fish in a new fish tank may also cause a rise in harmful ammonia and nitrites.

3. Add activated carbon media to the filter, whether loose or carbon pads.

Adding activated carbon media or activated carbon pads to the filter will help clear the water and adsorb nutrients that feed the bacteria bloom.

4. Seed the aquarium.

If you have access to another healthy, well-established fish tank, adding a few handfuls of gravel from that will seed the beneficial bacteria and speed up the clearing process.

Aquatic stores sometimes keep filter cartridges, bio-sponges and wheels floating to seed them with bacteria and will send these items home with new setups to help get the biological balance going. This has the same effect as adding gravel from an established tank.

5. Test your aquarium water.

Have it water tested for ammonia and nitrite as soon as the water begins to get cloudy. In most situations the levels will be zero, meaning there is no cause for concern.

Should I do nothing and let nature take its course?

  • Yes! Without question, doing nothing is the best approach for a new fish tank, as long as ammonia and nitrite levels are not on the rise. Cleaning the filter does nothing except disrupt the few beneficial bacteria that have had a chance to get established. These “good guys” will eventually outcompete the cloudy water bacteria for food, starving them out and breaking down their carcasses.
  • Water changes clear the water temporarily, but in a day or two the cloudiness reappears, often even worse than before. That’s because new water provides a fresh supply of nutrients, causing the cloudy water bacteria to populate even more.
  • Left alone, the cloudy water bacteria will eventually consume all the nutrients in the water and die out. This is part of the cycling process!

I understand that seeing cloudy water in a new aquarium, can be alarming. But the best advice is to be patient and wait it out. Don’t add any more fish, feed sparingly once every other day, have your water tested and just leave the filter alone for the time being.

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