How To Remove Hard Water Stains From Fish Tank (In 6 Steps)

Hard water stains are common in fish tanks due to the humidity and constant water evaporation. Evaporated water can leave behind a chalky residue on the glass surface which can become quite challenging to remove, especially over time.

You will need to take some basic steps to remove hard water stains from a fish tank, which I will briefly cover below.

  1. Remove some water from the tank and place it in a separate holding tank. This is where you will keep your fish, and by using water from the main tank, you will avoid shocking the fish.
  2. Remove your fish carefully with a net and place them into the holding tank.
  3. Remove all water using a gravel siphon or similar for heavily stained glass. If the stains are not too bad, you will not need to remove all of the water. Minor staining will require softer cleaning substances that will not harm fish if accidentally mixed with the water.
  4. Select the appropriate cleaning solution and allow it to soak into the stains; then, using a soft toothbrush, algae scraper, or similar, you can try to loosen the stains. More difficult stains will require you to repeat these steps.
  5. Wipe any remaining residue away with a dry cloth or kitchen towel.
  6. Refill your tank with the water you removed, and return your fish to their tank.

The cleaning process above may seem quite obvious, but removing hard water stains is not that difficult. Some variations are dependent on the type of tank you have, the level of staining, and the chemicals you will be using.

How To Remove Hard Water Stains From Fish Tank

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The rest of this article will cover specific cleaning solutions recommended when cleaning hard water stains from fish tank glass, and I will also cover the methods required for plastic and acrylic tanks.

Finally, I will provide a few extra tips to remove the most difficult stains and how to prevent calcium buildup in your fish tank.

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What Causes Hard Water Stains In A Fish Tank

Hard water stains in a fish tank are typically caused when mineral-rich water begins to evaporate. The evaporated water leaves the heavier minerals behind, attached to the glass surface, causing a white residue, which is usually a mixture of calcium and magnesium often called limescale.

Over time, as pure water evaporates, the minerals left behind will become more concentrated, making the stains thicker and more difficult to remove. Topping up the water in your tank is best done using a source of pure distilled water, as this will replace the pure water that has evaporated without adding extra minerals.

Hard water will usually be present in your fish tank if you live in an area with hard water or are using unfiltered water from a well or natural spring. You may also notice a white chalky buildup of limescale around your taps or kettle. The best way to determine if your water is hard is to use a test kit.

If you live in a hard water area, it is important to use a water conditioner when adding new water to your tank. This will remove chemicals such as chlorine to make the water safe for your fish, and it will also help remove many of the minerals present in the water.

What Causes Hard Water Stains In Fish Tank
Hard water usually comes from your tap and you will often notice calcium deposits or limescale around your taps.

How To Remove Hard Water Stains From Glass Fish Tank

If you have hard water stains on your glass fish tank, there are a few ways to remove them. Stains that have been left to build up over time will be more challenging to remove, whereas lighter, chalky stains will easily wipe away with a soft brush.

You can use a soft toothbrush or algae pad for scrubbing the glass. Don’t use stiff brushes or scouring pads as they can scratch the tank glass, causing permanent damage.

If the stains on your aquarium glass are a chalky, white residue, the methods for cleaning them will be much gentler on your tank and its inhabitants. Generally, I would recommend removing your fish as stated in the steps at the beginning of this article, however, light stains may not require you to remove your fish.

The cleaning solutions that you will be using do not contain any harsh chemicals, but they may alter the pH of your tank if they enter the water in large amounts. Drastic changes to pH can cause illness and shock to fish, so I would remove 3-5 inches of water into a bucket, giving you room to work. You can return the water after cleaning.

First, you can try using vinegar. Mix equal parts vinegar and water in a spray bottle and mist the affected areas. Let the solution sit for a few minutes, then wipe it away with a soft cloth. Using vinegar to clean a fish tank is a great choice but lemon juice is another alternative. Both are pretty acidic and will help lift the glass’s stains.

Hard water is generally quite alkaline with a high KH (Carbonate Hardness) and a strong buffering capacity against acids, so neither should drastically affect your pH in small amounts.

If Vinegar or lemon juice doesn’t work, you can try using a paste made from baking soda and water. Again, apply the paste to the stains and let it sit for a few minutes before wiping it away. Baking soda is a great oxidizing agent that will lift stains quite easily.

You can also scrape off thick limescale deposits, but you will need to exercise caution, or you may scratch the glass. A razor blade can help scrape away thicker deposits, however, you will need to hold it flat against the glass to avoid causing any damage.

Persistent stains may require a commercial hard water stain remover, and you should empty your tank of fish and water before using these more potent chemicals. These products are available at most hardware stores and can effectively remove stubborn stains. Whatever method you choose, be sure to rinse the tank thoroughly after cleaning to remove any residue that could harm your fish.

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How To Remove Hard Water Stains From Plastic Fish Tank

Plastic fish tanks are more fragile than glass, so you need to take extra care when cleaning them. Non-glass fish tanks are generally made of acrylic, a form of plastic. Acrylic is lighter, cheaper, and easier to repair, but it is also easier to scratch and damage.

How To Prevent Calcium Buildup On Fish Tank
You can find out the level of water hardness in your aquarium by using a test kit.

You can also use most of the methods above to remove hard water stains on acrylic fish tanks, except for a razor blade or excessively harsh chemicals.

Acrylic is much softer than glass, so scraping or rubbing with anything but the softest of brushes or cloths can cause scuffing and scratching to the glass. If you choose to use a razor blade or algae scraper, you not only risk scratching the tank, but the solid calcium deposits may scratch the tank as they are removed.

Plastic and acrylic are also more porous than glass, so the chalky residue may seep into the pores and become harder to remove. You may need to soak the stains for longer and use a fine bristled soft brush to remove these stubborn stains.

Because plastic is also more porous than glass, harsh chemicals may burn or distort the plastic or cause discoloration and misting to the surface. Caustic soda, sulfuric acid, and similar chemicals should be avoided or used at your own risk.

How To Prevent Calcium Buildup On Fish Tank

Softening your aquarium water, especially if it is excessively hard, will help to prevent calcium buildup on your fish tank glass, however, calcium and other minerals that are found in hard water can be beneficial to aquatic life and you would not want to remove them altogether.

Before softening your aquarium water, you should find out its hardness level with a test kit and find out the level of hardness that is suitable for your fish. If your water is too hard, you can read this article: How to soften aquarium water (Quick and easy solutions) for some tips.

Because the main cause of scale on your fish tank glass is water evaporation, you should check the water level frequently. You will need to replace any lost water. Replacing water will dilute the minerals, and you will want to add water with low mineral content, such as purified, distilled water. I have already explained that only water evaporates, and minerals will remain, so the concentration of minerals will increase over time.

It would help if you also considered using a hood or lid that will allow the water vapor to collect and eventually drip back into the fish tank, reducing evaporation.


Hopefully, this article has given you insight into why limescale and calcium deposits can build up on your fish tank glass. Although hard water stains can be found around your heater, filter, and don’t forget to clean aquarium decorations, they are most commonly found around the waterline. The humid and damp glass contains high levels of these minerals when they dry out.

Hard water stains are not too difficult to remove if done frequently, but over time they can become solid and almost impossible to shift without some heavy scraping which can damage the glass.

Acidic compounds such as vinegar and lemon juice mixed into a spray bottle will break down the scale deposits, making them easier to remove without causing any actual harm to your fish when used in small amounts. Extensive cleaning using commercial chemicals will require you to empty the tank, effectively starting from scratch.

If your hard water stains appear quickly and are not caused by the evaporation process, it may mean your water is excessively hard. There are ways to soften the water, but you will need to measure its hardness with a test kit first.

If you live in an area with very hard water, you will need to treat the water before using it in your tank, or you can use distilled water, although water that is too soft and lacking in minerals is also not a good solution. Live plants and fish thrive on the minerals found in water, so you need to find a healthy balance.

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Jon O'Connell

I have kept both marine and freshwater fish and set up almost 100 aquariums. Although I am no longer doing it professionally, I hope I can help others to enjoy keeping happy, healthy fish by sharing my many years of experience.

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