Why Is My Fish Tank Getting Dirty So Fast?

Fish keeping can bring so much pleasure, and the beauty of a well-kept tank is like a gleaming jewel full of life. But the downside can be keeping on top of tank maintenance. If you often look at your aquarium and see murky water or unwanted algae, you may wonder: “Why is my fish tank getting dirty so fast?!”

The simple answer: Because it needs more or better cleaning!

Fish tanks require regular maintenance, such as weekly water changes, to stay clean. Ensure your tank has more than adequate filtration, is not overstocked, or is over-lit, and avoid giving your fish too much food. Maintaining a weekly cleaning routine will prevent your fish tank from becoming dirty. Keep reading for details on exactly what you need to do and why.

So, Why Is My Fish Tank Getting Dirty So Fast?

When I began keeping fish as a teenager in the 80s, learning how to maintain my fish tank in good condition, and prevent it from getting dirty, was a process of trial and error, and there was less information readily available.

Even the equipment we use for tank maintenance and cleaning has improved. I’ll explain the possible reasons why your tank is getting dirty quickly and how to avoid these. I’ll also explain water changes and recommend some of my favorite tools for helping you clean your tank.

Your fish tank is a nearly self-contained world; to maintain a healthy system, you need space, filtration, and the right number of suitable fish and plants. You’re creating a mini-ecosystem, and getting the balance right makes it easier to maintain tank cleanliness.

It’s much easier to maintain a healthy ecosystem in a larger tank. Smaller tanks are more prone to getting out of balance in terms of water quality. This leads to algae blooms, chemical build-up, cloudy water, and fish and plant die-off.

Even a big tank with excellent filtration is not a completely closed system, and daily, weekly, and monthly maintenance will help prevent a dirty tank.

You’ll need to look at the main things that cause tanks to get dirty quickly:

  • Fish tank size
  • Filtration
  • Lack of routine maintenance like water changes
  • Lighting
  • Over-stocking
  • Overfeeding
  • The fish species

Fish Tank Size

It’s much easier to maintain healthy biological levels in a larger tank. In a small tank, even slight changes can affect the quality of the water. Fishkeepers new to the hobby should avoid small tanks and fishbowls and get the largest tank they can afford to buy and run.

At a minimum, try to get at least a 20-gallon tank, as it is easier to maintain the water chemicals at the right levels than with small or nano-tanks. The more water in a tank, the more stable the temperature and ammonia and nitrate levels will be.

A larger tank doesn’t mean you can fill it up with more fish, and we’ll discuss how to stock your fish tank to ensure you don’t overload the tank system.


Smaller tanks (usually under 30 gallons) will use internal or power filters, while larger tanks use more powerful external filters. You must ensure that the filtration system you use is powerful enough for both the size of your tank and the kind of fish you are keeping.

Filters perform filtration in two ways – mechanical and biological. Mechanical filtration is the process that cleans out sediment and larger particles of dirt from the water passing through the filter medium.

Biological filtration utilizes bacteria in your filter to convert ammonia and nitrites to nitrate. These are an essential part of your filtration process, and a new tank must be cycled first to set up healthy levels of bacteria to combat rising ammonia.

Part of your essential tank maintenance is cleaning your filters. You want to remove the built-up sediment without killing the bacteria, which is why you rinse the filter medium clean with running water and don’t use chemicals to clean it.

If your filter is not powerful enough for the size of your tank and the filter media is not kept clean, your tank will quickly become dirty.

Lack Of Routine Water Changes

Even larger tanks with excellent filtration systems will still need partial water changes. Removing a percentage of the tank water weekly and replacing it with fresh, conditioned water will help keep the water clean and ammonia levels lower.

Smaller tanks will need water changes more frequently, while larger tanks can take 25% water changes weekly without worry. But two important rules:

  • Never use chlorinated water directly from the tap.
  • Make sure the treated water reaches the correct temperature before adding it.

One of the simplest ways to change the water while keeping the tank’s substrate clean is to use an aquarium gravel vacuum to siphon dirt. The vacuum sucks particles of dirt, fish waste products, and uneaten food where it has fallen between the gravel and removes dirty water.

Some fish species are particularly messy eaters, gravel diggers, and waste producers and may require more frequent water changes.

Lack of weekly water changes will result in a tank quickly becoming dirty and an unhealthy environment for your fish.


Most tanks require lighting for you to see your fish and to allow aquarium plants the resources to thrive and grow. If you have no plants or only plastic ornamental plants or rocks and wood, the light may cause algae blooms.

You can also have algae blooms in planted tanks with the wrong kind of blue lighting or too much lighting. A tank kept in bright sunlight may also experience an overgrowth of algae. Too much bright light, an overabundance of nutrients, and a lack of algae grazing fish will lead to more algae.

You can add more plants, ensure the right lighting, scrape down algae on the glass, or get an algae-eating species to help control algae that dirty the tank.

Why Is My Fish Tank Getting Dirty So Fast?


If your fish tank is getting dirty after one day, you have likely overstocked your tank. Too many fish will lead to an excess of waste, higher levels of ammonia, and cause disease to run rampant.

An overstocked tank will also result in unhappy fish, stress from lack of oxygen, and bullying from crowded conditions. It is far better to have an understocked, over-filtrated tank than the opposite.

When choosing species for your tank, keep them to appropriate numbers and species that live well together. Some species of fish need a lot of space, while others are more comfortable in small shoals.

A very rough rule of thumb is the adult size in inches per gallon. In this case, a minimum size tank of 20 gallons will support roughly eleven neon tetras. This is another reason why larger tanks are better for fish keeping.


Beginners in fish-keeping often overfeed their fish, creating more waste that leads to dirty water, ammonia spikes, and algae blooms.

Avoid feeding fish more than they can consume in about 2-3 minutes. Rather start small and feed them more than to over-feed them.

The Fish Species

The type of fish you keep will also determine how quickly your tank gets dirty. Some fish are just messy!

One of the biggest culprits of messy tanks is the goldfish. Goldfish produce a lot of waste for their size and tend to destroy plants in the tank.

Other fish notorious for producing waste are Oscars, plecos, and clown loaches. When keeping messy fish, you should avoid overstocking and use more powerful filtration to keep the water free of debris.

Smaller fish like zebra danios, neon tetras, diamond tetras, and white cloud mountain minnows will generally produce less waste.


There are multiple factors that might be contributing to your fish tank becoming dirty too fast. Choosing the right fish, performing regular gravel siphoning and water changes, and keeping filters clean will help prevent your tank from becoming dirty too quickly.

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