How to Adjust Water Chemistry in Aquarium

Few aquarists have tap water which is ideally suited to the fishes they wish to keep. There are however steps that can be taken to modify this to create the correct conditions.

Hardening aquarium water is generally a straightforward matter of introducing calcareous (i.e. CaC03-rich) decor materials such as tufa rock and coral sand. Proprietary salts are now widely available to replicate the chemistry of hard water lakes such as Lakes Malawi and Tanganyika. These are not ‘marine’ salt formulations, which are unsuitable; ‘salt’ is here used in its chemical sense (e.g. sulphates, chlorides, carbonates etc.).

Softening water

Aquarists in hard alkaline water areas who want to keep or breed species that prefer soft acidic water, will need to soften their water before use. This is not as simple as increasing the hardness. A number of methods, of variable efficacy and requiring variable injections of money or effort, are available.

aquarium chemistry How to Adjust Water Chemistry in Aquarium

Water Softening Resins

Proprietary water softening (ion exchange) resins are available, hard water being passed through the resin to emerge as soft. Unfortunately, during this process calcium (Ca) ions are usually exchanged for sodium (Na) ions, so although the resulting water is soft, it is still mineral-rich and unsuitable for most ‘soft water’ species.

Rain Water

Alternatively, the aquarist can collect rain water, which is mineral depleted, usually has a neutral or slightly acid pH, and can be easily influenced to become more acidic. The disadvantage of this method is air pollution from the emissions of cars, industry etc., which contaminates the falling rain and renders it quite unsuitable for fish!

Unless you are certain that rain water in your area is free of, or low in, airborne contaminants, it may be better to forego its collection, although rain collected well into a lengthy heavy downpour, after the air has been ‘washed’ clean, is usually safe to use.

It should be collected only from roofs which will not affect its chemistry or quality, and gutters should be kept clean. Ideally it should be strained through activated carbon to remove any residual contaminants.

Rain water can be used ‘neat’ or to dilute the hardness of tap water.

Pure H20

The safest and most effective method of providing soft water is to use water which has been distilled, de-ionized, or passed through a reverse osmosis (RO) unit. Each method removes all contaminants leaving pure H20, which is normally mixed with at least some tap water in order to provide a minimal mineral content, and stabilize the pH. Remember that all contaminants arc removed, including the free oxygen that fishes breathe, so the water must be aerated before use or they will literally suffocate.

In addition to removing mineral stilts, all three methods remove organic contaminants – useful if your tap water quality is less than optimal.

Distilled Water

Distilled water can be obtained from pharmacies and laboratory supply companies, but it is expensive and rarely used for aquaria.


De-ionizers normally use two ion-exchange resins; one exchanges anions (negatively-charged ions), and the other cations (positively-charged ions) in an aqueous solution, removing almost till organic and inorganic substances. While initially very efficient, both resins are quickly exhausted and require frequent recharging or replacement.

Reverse Osmosis (RO)

In recent years this new method of purifying tap water has found great favour with marine aquarists because of its ability to remove in excess of 95% of all substances held in solution, including pesticides, bacteria, heavy metals and dissolved organics. It utilizes a thin film composite (TFC) membrane, through which only water molecules can pass. All other molecules are excluded and the result is water as pure as could be expected from a cloudburst over the mountains.

Hard water may also be partially softened by boiling. This removes temporary hardness (caused by bicarbonates), but permanent (CaCO3) hardness remains.

It is essential to avoid using decor items which will add hardness to your hard-won soft water.

Modifying pH

Increasing pH is normally simply a matter of adding calcareous material to the aquarium. Bicarbonate of soda, added in solution in small doses (you need to test the tank pH between doses), can also be used, especially if a quick result is required.

Acidifying the Water

Reducing the pH of soft water (trying to reduce that of hard water is a waste of time, as it will revert as quickly as you can modify it!) is best achieved by peat filtration until the target is reached, although water changes will probably mean that some degree of peat filtration is constantly required.

Some types of aquarium peat are more ‘active’ than others, some reducing pH slowly whilst others make it drop dramatically. Obviously the latter must not be used in an aquarium containing fish. Only pure peat should be used; horticultural moss peat is acceptable – and cheap – but only types without additives such as fertilizers should be used.

Alternatively, pack a nylon bag (you can use pantihose) with peat and soak it in a bucket of soft water; the tannins it contains will leach into the water and the resulting extract can be added to the aquarium water as required. You can also boil up peat in water (avoid any pan, e.g. those made of aluminium, which is likely to cause metallic contamination), to provide peat extract. In both cases add a little at a time and test the tank pH after each dose.

Proprietary products of a similar nature are available but these are usually considerably more expensive.

Chemical pH Adjusters

It is also possible to purchase proprietary chemical pH adjusters. These must always be used in accordance with the manufacturer’s instructions, and even then there are sometimes unexpected side-effects. Natural methods of adjusting pH are safer and thus preferable.

pH Buffering

The metabolic byproducts of fish and plants have a constant slight acidifying effect, so it is wise to monitor the pH regularly to ensure that it doesn’t drop suddenly or too far. It is wise to include some calcareous decor in hard water tanks to ‘buffer’ the pH against this gentle acidification – as acids are produced they dissolve the CaC03 and the pH rises again.

The pH of extremely mineral-poor water can be very unstable and fluctuate widely, a state of affairs not generally enjoyed by its occupants. Again the answer is to include a small amount of calcareous material as a stabilizing buffer.

Creating salt or brackish water

Only specially formulated ‘marine salt’ should be used for this purpose, mixed with good quality fresh water (ideally RO) according to the manufacturer’s instructions. Domestic salt may contain harmful additives and is not suitable.

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