Should Treated Water Be Used in an Aquarium? The Ultimate Guide to Water Preparation

One of the most frequently asked questions by those new to the aquarium hobby and by professionals seeking to improve the aquatic ecosystem is undoubtedly:‘Should treated water be added to the aquarium?’ Most aquarists intuitively know that tap water can be dangerous for fish; however, it is often overlooked that even crystal-clear water from a water purifier can harbour hidden dangers when used directly. The secret to establishing a successful aquarium ecosystem lies not only in the water being ‘clean’, but also in it having a chemical balance suited to the biological needs of the organisms within it. In this comprehensive guide, we will examine, step by step, how to use filtered water safely in aquariums, which minerals are required to protect fish health, and how to achieve ideal water parameters.

Why Is Tap Water Dangerous for Aquariums?

The tap water that reaches our homes undergoes various disinfection processes to remove bacteria and viruses that could pose a threat to human health. Whilst these processes are essential for public health, they can have devastating consequences for the closed ecosystem of an aquarium and for aquatic life with sensitive gills. The heavy metals and chemicals present in tap water place pressure on the natural biological cycle, paving the way for fish losses.

The Deadly Effects of Chlorine and Chloramine

Chlorine is the most commonly used disinfectant in municipal water treatment plants. Chlorine causes respiratory failure by burning the gill tissues of fish. As a traditional method, leaving water to stand in an open container for 24–48 hours may be sufficient to allow free chlorine to evaporate; however, chloramine is now used in many water supply systems. Chloramine is an extremely stable compound formed by the combination of chlorine and ammonia, and it does not evaporate even if the water is left to stand for days. The most reliable way to remove this hazardous compound from water is to use a water treatment system fitted with high-quality carbon block filters or to opt for specialised chemical water treatment products.

Can Treated Water (Reverse Osmosis – RO) Be Used Directly in an Aquarium?

Water purification systems, particularly reverse osmosis (RO) systems, purify water at a cellular level, effectively turning it into a ‘blank canvas’. These systems filter out all harmful particles, heavy metals and sediments found in tap water. However, this thorough purification does not mean that adding the treated water directly to the aquarium is either sufficient or the right course of action.

Reverse osmosis filters work by passing water through a semi-permeable membrane under high pressure. According to data from Freshwater Systems, an RO system filters out between 92 per cent and 98 per cent of the calcium, magnesium, chloramine and other dissolved substances present in the water. Although the water produced by this process is of excellent purity for human consumption or industrial use, it is completely devoid of the vital minerals that fish require for bone development and osmoregulation (water and salt balance).

Furthermore, these systems produce a certain amount of waste water whilst carrying out the purification process. According to reports by the United States Geological Survey ( USGS), standard RO systems can produce between 2 and 4 gallons of waste water for every 1 gallon of purified water, depending on their design. This is a technical detail that must be taken into account when calculating water costs, particularly for high-volume aquarium set-ups.

Osmotic Shock and Mineral Deficiency

Pure water, completely stripped of its minerals, tends to pass through cell membranes very rapidly. If you place a fish directly into a tank filled with 100 per cent pure water (distilled water), a massive pressure difference arises between the fish’s body—which has a high mineral concentration—and the mineral-free water outside. This situation creates a state of ‘osmotic shock’, causing water to enter the fish’s cells uncontrollably and leading to the cells bursting. Fish constantly absorb minerals such as calcium and magnesium from the water via their gills; a deficiency in these elements causes stress in the short term and, in the long term, skeletal deformities and death.

What is Remineralisation?

The golden rule for using treated water safely in an aquarium is remineralisation. To RO water that has been purified and stripped of all harmful substances, mineral salts must be added externally in a controlled manner, in proportions appropriate to the natural habitat of the species you are keeping. This way, you not only remove chlorine and heavy metals but can also stabilise the water’s hardness (GH and KH) at exactly the level you require.

Water Resources Comparison Table

Preparing Aquarium Water According to the Type of Fish

Each species of fish originates from rivers, lakes or oceans in the wild, each with its own distinct characteristics. Consequently, the concept of ‘ideal aquarium water’ varies considerably depending on the species you are keeping. Using reverse osmosis (RO) water gives you the freedom to fine-tune water parameters to match the natural habitats of these creatures with millimetre precision.

Freshwater Aquariums (Discus, Cichlids, Japanese)

Experts generally accept that the pH tolerance range for mixed freshwater aquariums is between 6.0 and 9.0. However, this range narrows when considering specific species. For example, Discus fish from the Amazon river system prefer soft, acidic water (pH 6.0–6.5), whilst cichlid species from Lake Tanganyika or Lake Malawi in Africa require quite hard, alkaline water (pH 8.0–8.5). Aquarists wishing to set up an Amazon-themed blackwater biotope must use treated water, as they will be unable to overcome the high carbonate hardness (KH) found in tap water. Adding redwood cones or catappa leaves to RO water with a KH value of zero easily creates an acidic effect on the pH balance, bringing the water to the desired dark colour and low pH level.

Shrimp Aquariums

Shrimps of the Caridina and Neocaridina genera, in particular, are extremely sensitive to water quality and heavy metals (such as copper). Even trace amounts of copper that may be present in tap water can be fatal to a shrimp colony. For this reason, professional breeders invariably use purified water obtained from reverse osmosis systems for their shrimp aquariums. Once the purified water has been obtained, only remineralisation salts specifically produced for shrimp are added to the water. According to research, sensitive shrimp species require water prepared using treated water, with a conductivity level stabilised within the range of 200–500 µS/cm. This delicate balance is vital for moulting and spawning processes.

Marine and Reef Aquariums

The use of reverse osmosis water in saltwater and reef aquariums is not a matter of choice, but an absolute necessity. Synthetic aquarium salt mixes used in marine aquariums contain all the minerals found in seawater in the correct proportions. If you mix this salt with tap water, which is rich in minerals, the calcium and alkalinity levels will rise to dangerous levels. Furthermore, the silicates and phosphates present in tap water lead to irreversible algal blooms in marine aquariums. For this reason, reef aquarists always start with 100 per cent RO water.

Table of Ideal Water Parameters by Species

Preparing Aquarium Water: Expert Tips

Although water parameters may seem complex in theory, with a few practical expert tips, you can achieve the ideal water quality in a home environment without disrupting the biological cycle. The right method should be chosen according to your level of experience and the sensitivity of the animal you are keeping.

Regular maintenance is essential to maintain the stability of the aquarium ecosystem and to remove toxic nitrogen compounds such as nitrites and nitrates. Experts recommend carrying out a water change of between 10 per cent and 20 per cent of the aquarium’s volume each week using treated water, in order to prevent ammonia spikes and maintain stable water quality. The preparation of the water to be used during these changes is of great importance.

Beginner Level: 50/50 Mixing Method

If you are not yet familiar with using sensitive test kits and are keeping hardy freshwater fish, the safest method is the mixing method. With this method, 50 per cent of the water added to the aquarium comes from purified water from a water filtration system, whilst the other 50 per cent is tap water that has been treated with a high-quality water conditioner to remove chlorine and heavy metals. In this way, whilst benefiting from the purity of the filtered water, you also achieve free remineralisation by utilising the calcium and magnesium minerals present in the tap water.

Advanced Level: 100 per cent RO water and mineral salts

Professionals and breeders of sensitive species do not want to leave anything to chance. This method uses 100 per cent reverse osmosis-treated water directly. Commercial aquarium salt (remineraliser) is added to a bucket of purified water until the target pH and GH values are reached. Once the mixture has been thoroughly aerated, it is verified using TDS and pH tests; when it has reached the same temperature as the aquarium, it is added to the main tank.

Equipment Required for Measuring Water Parameters

It is impossible to understand the chemistry of water by sight alone. When working with treated water, there are some essential measuring tools that aquarists should have to hand in order to reduce the margin of error to zero.

A TDS (Total Dissolved Solids) meter is the most practical way to check whether the filters in your water purification system are working properly. The TDS value of purified water should be between 0 and 15 ppm; if this figure rises, it is time to change the filters. Whilst pH meters are used to determine the acidity or alkalinity of the water, a GH (General Hardness) test kit must be used to measure the levels of calcium and magnesium you add to the water, and a KH (Carbonate Hardness) test kit (Carbonate Hardness) must be used. These drop tests are the only reliable indicators of whether the remineralisation process has been successful.

Choosing a Water Purifier for Optimal Water Quality

You should be careful when choosing a device for your home that will both meet your family’s need for healthy drinking water and provide the perfect infrastructure for your aquarium. Whilst standard three-stage filters do remove chlorine, they do not provide full reverse osmosis. The system you are looking for must be a genuine Reverse Osmosis (RO) system featuring a membrane filter.

Some modern systems offer additional safety measures after purifying the water. UV filters integrated into RO systems eliminate 99.9 per cent of bacteria and viruses in the purified water, ensuring maximum safety for your fish. However, the mineralisation stage must not be overlooked, even in models with UV filters. If your unit features an alkaline or mineral filter as the final stage, it is advisable to carry out a pH test beforehand to ensure the water’s pH level is suitable for your fish.

If you’re looking for a professional solution that completely removes chloramine thanks to long-lasting carbon block filters, water purification systems —with their high filtration capacity—are an excellent investment for both your household and your aquarium hobby. Reliable brands offering services such as free water analysis help you choose the right filtration system by scientifically measuring the quality of your water.

Furthermore, if you wish to enjoy the convenience of both hot and cold water in an office environment or a large home, whilst also having easy access to purified water, these stylish water dispensers with an integrated system will make your life easier. You can easily mineralise the pure water you obtain from here in a bucket and safely add it to your aquarium.

Frequently Asked Questions

Can fish live in treated water?

Fish cannot survive directly in treated water (distilled water). Whilst water purification systems filter out harmful substances, they also completely remove beneficial minerals that are vital for fish bone development, cellular functions and osmoregulation systems. Fish placed in 100 per cent pure water may suffer ‘osmotic shock’, causing damage to their cells. The process of mineral restoration (remineralisation) is absolutely essential for the safe use of purified water in aquariums.

Treated water or tap water?

Neither should be added directly to the aquarium on its own. Tap water contains chlorine, chloramine and heavy metals that can poison fish; treated water, on the other hand, is an excessively pure water that has been stripped of essential minerals. The ideal solution is to enrich the purified water with special mineral salts purchased separately, or to achieve a balance by mixing it half and half (50 per cent purified, 50 per cent tap) with tap water from which the chlorine has been removed using water-conditioning chemicals.

What sort of water should be used in an aquarium?

You should use water that has pH, conductivity (TDS) and hardness (GH/KH) levels suitable for the natural habitat of the animal you are keeping, and which is completely free from chlorine, chloramine and heavy metals. The most suitable options are dechlorinated, conditioned tap water with a water conditioner added, or treated water in which the mineral levels have been adjusted using aquarium salts.

Should you use bottled water in an aquarium?

Yes, bottled drinking water in jugs or PET bottles can be used, but you should always read the label carefully. As bottled water is disinfected during production, it is chlorine-free; however, pH levels vary widely from brand to brand, ranging from 6.5 to 8.5. Not every type of bottled water may be suitable for every species of fish; for example, a bottled water with a high pH could cause stress for Discus, which prefer acidic water, whilst a water with a low pH could cause stress for Cichlids, which prefer alkaline water.

Copy Blog Link