Using Pure Water in the Aquarium

Pure water can have specific uses in the aquarium, including:

• Topping up water lost to evaporation. This should ensure a constant hardness level in the aquarium, as only pure water evaporates, leaving behind salts, etc. This means that constantly topping up with tapwater will gradually increase the hardness/mineral content of the water because pure water is being replaced with water containing a range of minerals.

• Lowering hardness (and usually pH at the same time) by mixing a percantage with your normal tapwater.

• Providing a much purer source of water for your fish, which can have enormous benefits.

Note that pure sources of water must never be used alone in the aquarium. They are too pure, and contain none of the essential minerals that fish need, and also have no buffering capacity to stabilise the pH. It can either be mixed with a certain percentage of tapwater (it is possible to produce various hardness levels in this manner), or the commercially available salt mixes can be used to replenish a balanced mix of electrolytes - note that these are not the same as normal 'aquarium salt'. The following paragraphs outline some of the sources of 'pure' water.

Distilled water is produced by causing water to boil and turn to steam, which is then cooled (condensed) back into liquid water and collected. This is normally carried out in laboratories and industry, and is not really suitable for the home aquarist to perform. Distilled water is often available bottled however. It may be practical to use it if the required amount is small.

Reverse osmosis (RO) is a process which purifies water by forcing it through a fine membrane, which leaves impurities behind. This is probably the most practical option for the hobbyist, especially if larger amounts are required, i.e. for multiple tanks.

Deionised water (DI) is produced using special materials which attract ions from the water and bind them to their surfaces. Note that this is not the same as ion-exchange, where one ion is swapped for another, as in domestic water softeners. DI units can produce very pure water, but they are generally expensive to use. The cartridges require frequent replacement, and the amount of pure water produced per cartridge will decrease for tapwater which contains more minerals and contaminents. DI can be useful when included as the final stage on a reverse osmosis unit, where the combination can produce very high purity water.

Rainwater is in theory pure, because it is water which has evaporated and then condensed, similar to distillation, but on a planetary scale! However, a lot of rainwater is likely to be contaminated from the pollution in our atmosphere. If you live in a largely unpolluted part of the world, and decide to use rainwater, there are a few things which should help to ensure it is as pure as possible:

• Do not collect water after a long dry spell, there is likely to be a higher-than-normal amount of contaminents from the atmosphere. Collect after an initial rainfall has 'cleaned' the air to some extent.
• Collect the water from a clean roof or other source, and store in clean well-aerated containers.
• Filter the rainwater through carbon as an extra precaution - carbon will remove a wide range of toxins and impurities.

Note that spring water, mineral water and other bottled waters can vary widely in their composition, depending on the source, and they are not a pure source of water. They can vary from soft and acidic to hard and alkaline, and in some cases may be no more suitable for your fish than your tapwater or other source! It is also likely to prove expensive for larger amounts. Check the analysis on the bottle if you decide to use bottled water, and/or check the basics with your own test kits.

A few additional points to remember:

• Pure water can leach substances from the containers in which it is stored. It should therefore be stored in clean non-metallic containers, such as glass tanks or high-grade plastic which will not leach colour.

• Freshly purified water should be aerated for at least an hour, or left standing overnight, before use - as the oxygen content will be very low or essentially zero initially.

• Test kits for pH and hardness are essential if you use modified pure water for your aquarium.