Why is My Fish Tank Water Yellow? – 5 Reasons & How to Treat

why is my fish tank water yellow

Many beginner aquarium hobbyists ask, why is my fish tank water yellow? Although there can be many reasons, the most common causes of aquarium water turning yellow are tannins, diatom algae effects, organic compounds, and excess bio-load.

Please continue reading to learn details about these yellow aquarium water causes and how to fix them.

Why Does Aquarium Water Turn Yellow or Brown?


Beginner aquarists often scratch their heads, wondering why the aquarium water yellow or brown when it should be clear. Their concern is understandable. After all, it would be challenging to see fish’s vibrant colors in a hazy aquatic environment.

So, what causes fish tank water to lose its clarity and turn yellowish or brownish?

1. Fish and food waste


Overstocking is one of the most significant causes of an increased bio-load. Although you may stick to the one-inch-per-gallon rule, a crowder tank means it’ll be quicker to be filled with poo.

Feeding more food than the recommended amount is another reason for the unpleasant color in your tank. Overfeeding can lead to leftovers, which ultimately go bad and decay, further increasing the bioload.

Although you will not see fish turning yellow and dying immediately, increased bioload can make your fish sick and may eventually die of ammonia toxicity.

2. Decaying plants


Like fish, plants comprise organic molecules. Although live plants can make an aquarium look more stunning, neglect can lead to plant death.

A dying aquatic plant should be easy to spot. The leaves turn from green to yellow before they become brown and fall to the aquarium substrate. The issue is when the fish tank has many plants, making identifying a dying plant part challenging.

Decaying plants can turn the aquarium water brownish or yellowish. They also release organic compounds into the water, increasing the fish tank’s bioload.

So, too many fish and plants can lead to increased nitrogenous waste, answering the question of why is the fish tank water brown or yellow.

3. Tannins in driftwood


Seasoned aquarists love placing driftwood in their tanks to ensure a healthy fish population while improving the aquarium’s overall aesthetics. Driftwood also releases tannins necessary for acidifying the watery environment for tropical fish to thrive.

Although beneficial to fish, tannins can discolor the water. Tannins make the aquarium water clear but yellow or brown. It is worth noting that commercial tannins are light brown to pale yellow. Unsurprisingly, driftwood-released tannins can have a similar color.

Tannins might turn the aquarium water yellow, but their effects on fish are beneficial.

4. Tap water


Even after the chlorine is eliminated in order to be safe to fish, tap water can still contain other minerals such as tannins, manganese, and iron. Over time, these compounds can turn your water into a yellowish tint.

There’s also the issue that tap water may not be completely clear, to begin with. Although imperceptible to the naked eye, water from taps can have a yellow hue, which is often the result of rusty pipes or heating units.

5. Uncontrolled algae growth


Another cause of aquarium water yellowing is the overgrowth of brown or yellow algae in fish tank units.

Yellow algae are common in freshwater aquariums. These organisms look like sand or dirt in the tank and proliferate in warm water temperatures.

Meanwhile, brown algae are saltwater and freshwater diatoms. You will not see these organisms unless the water has high levels of nitrates and phosphates (a state common in high bioload).

How Do You Fix Brown or Yellow Fish Tank Water?


So, everything boils down to excess organic compounds in the water. What do we do now? Hence, it is crucial to address this issue to get rid of yellow water in fish tank units.

1. Do not overstock your aquarium.

Although the 1-inch-fish-per-gallon rule applies, you must also consider other fish stocking factors. Water quality (including pH and temperature) and fish size are crucial. But so is the feeding rate.

Hence, we advise beginner aquarists to research their fish very well, including feeding behavior. We cannot help but overemphasize that the amount of food will have a big say in how much excretion there will be in the tank.

2. Invest in a good-quality fish tank water filter.


These devices screen the aquarium water for particles, including dirt, uneaten fish food, plant materials, fish poo, and other substances. It is one of the best ways to get rid of brown water or yellowish fish tank water.

For instance, activated carbon charcoal filters are perfect for removing odors and dyes, giving you a pristine-looking aquarium. You can also find products that use biological agents to gobble up everything, including dissolved organic compounds.

Some fish tank filters use mechanical filtration, “sifting” the water as it passes through sponges, filter floss pads, or filter socks.

We recommend a combination of mechanical, biological, and chemical filtration to address a wide range of water quality issues.

However, if you notice the water yellowing persists despite installing a water filter, you might want to check the device’s components. Clogged or poor-quality filter membranes cannot do their jobs properly.

3. Remove algae.


It is worth noting that brown algae proliferate with increased nitrogenous waste in the water. Hence, addressing the cause of excessive organic compounds should also limit algae growth.

You might also want to reconsider removing them because algae benefits fish and will not harm them.

Should you decide to remove them anyway, install a UV sterilizer filter, which kills algae and water borne microorganisms, including bacteria.

Alternatively, adding aquarium salt will make the water inhospitable to algae, killing them. Introducing algae feeders (i.e., Trachus snails and Amana shrimp) into the fish tank community is worth trying.

4. Partially clean the fish tank.

Dirty fish tank water, regardless of discoloration, requires partial cleaning. We do not recommend emptying the aquarium of water unless you want a fresh start.

We recommend staggered cleaning. For example, you can replace ten percent of the dirty water this week and perform another water change the following week.

This technique might seem tedious, but it gives your fish a better chance of adapting to the water quality changes. Perform this method until you see no more yellowing in the aquarium water.

Frequently Asked Questions


Can bacteria cause yellow water?

No, bacterial overgrowth will not result in yellow water. However, it’ll still affect the color and look of your tank by turning the water brown and cloudy. When this happens, you need to change the dirty aquarium water immediately.

Is yellow water good for fish?

Yellow or brown water in aquarium units is not always harmful to fish; you’ll have to look into the cause first.

For instance, tannins can give aquarium water a yellowish or brownish tinge. Although these substances can soften the water by lowering the pH, some fish thrive in a slightly acidic environment. Examples are South American species.

Meanwhile, yellowing due to dissolved organic compounds (i.e., decaying plant materials, decomposing fish, uneaten food, and fish poo) can be problematic for fish by changing the water’s ideal chemistry. It can lead to ammonia poisoning.

Tips to prevent yellow water in fish tank

The following tips can prevent yellow water in a fish tank and keep fish tank clear.

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  • Ensure an optimally-working water filtration system guaranteeing more efficient waste removal from the fish tank.
  • Change ten percent of the aquarium water periodically to prevent nitrate and phosphate levels from increasing.
  • Give your fish food they can eat in two to three minutes. You can always add if they consumed their food in less than two.
  • Remove dead fish and decaying plants as soon as you notice them.
  • Treat your driftwood to remove as many tannins as possible.



Unmitigated organic compounds remain the top answer to “why is my fish tank water yellow?” It can stem from overstocking, faulty fish feeding practices, unhealthy plants, and poorly functioning or absent fish tank water filter.

Other potential causes include tannins from untreated driftwood, bacterial overgrowth, excessive algae activity, and tap water use.

Removing and controlling organic waste in the fish tank can help address water yellowing. Periodic water changes, a good filter, and observing best practices should help prevent such problems.