Water that is used for evaporation top off or for making artificial saltwater should be as pure as possible. Any purities present in it, may cause issues with the livestock within the marine aquarium. This is especially important for water that is used for evaporation top off, as even trace amounts in the water can add up to a significant amount in the aquarium system as it is added constantly.
The options available for purifying water for use are discussed below.
Untreated Tap Water
Typically, water straight from the tap will not be suitable for long term use in the aquarium. Local water authorities deliberately add many components such as chlorine and chloramine to tap water to maintain its microbiological safety, and flocculants for clarification to render it suitable for domestic household use. In its journey to the household, water also picks up other contaminants such as copper from pipes. Unfortunately whilst safe for human consumption, it is not necessarily good enough for use in aquaria.
It is not recommended that untreated tap water be used in the preparation of synthetic salt water mixes, or for top-up replacement of evaporative losses.
Aged Tap Water
At its simplest, if the quality of tap water is high to start with, water purification can entail aging the water to enable gaseous components to be released by tapwater. This is effective in removing chlorine, but does nothing to address removal of disolved salts or nutrients. 24 hours aging may be sufficient, and this can be facilitated by aerating the water with an airstone to ensure effective gas exchange.
Various bottled solutions are available from aquarium stores that enable tapwater to be used straight away. These products are said to neutralise chlorine and chloramine as well as binding up metals such as copper. Whilst treating these components to render them supposedly safe, they are still left in the water, and are free to accumulate in the aquarium over time.
Rainwater can be used, however it is strongly dependent on the way in which it is collected. Water collected from building rooves is likely to be contaminated with pollutants such as bird droppings, dust and road grime. Additionally rainwater, particularly in the first stages of a rain downfall, can contain significant amounts of dust and pollutants from the atmosphere.
Reverse Osmosis (RO)
Reverse osmosis filters are devices in which water under pressure is forced through a semi-permeable membrane and in doing so are capable of removing disolved salts.
RO systems take a single water feed from the tap and produce a purified stream that has had contaminents removed, as well as a wastewater stream in which the contaminants have been concentrated. Typical household units produce around 2-4 litres of wastewater for every litre of purified water. The waste water can be used for washing clothes or watering plants, but can be quite wasteful if it is discarded to the drain in areas where water restrictions apply.
There are various membranes available, although TFC or Thin Film Composite membranes appear to be the most appropriate for aquarium applications, and are capable of reducing contaminants to very low levels of Total Dissolved Solids (TDS).
Deionisation entails the use of canisters of resin beads which are capable of removing dissolved ionic species from the feed water. Two resins are used - one to attract and remove positive ions (cations) and another to remove negative ions (anions). The two resins may be contained in seperate canisters or may be blended into a mixed bed resin. The benefit of seperating the resins is that they may be regenerated once they are exhausted to enable the resins to be reused. DI filters do not produce any waste water and purify 100% of the water that passes through them.
DI can be used on its own (eg Aquarium Pharmaceuticals Water Purifier) or may be used as a final stage to further treat the purified water exiting a reverse osmosis filter. (Referred to as an RO/DI system). RO/DI systems enable much longer life for the DI resins and are capable of reducing contaminants to extremely low levels.