Water Changes in Aquariums
There are no hard and fast rules about how often a water change should be carried out, or how much
water should be changed - every tank setup is different. There are, however, some sensible guidelines which
can be followed to keep fish and plants healthy.
- Water changes should be carried out regularly - 25% per month should probably be considered a MINIMUM
for a lightly stocked community tank.
- Some species or tank setups will require more frequent water changes to maintain the water
quality. For example, Malawi cichlid tanks, which are often well stocked to spread out aggression,
would probably benefit from a 30% water change at least every two weeks.
- More frequent changes will be beneficial when breeding and raising fry. Much improved growth rates are
reported where very frequent water changes have been carried out.
- If a tank is fully stocked, the high waste load will necessitate larger and/or more frequent
water changes than an understocked tank. Also, messy feeders - such as large cichlids - will cause
the water to deteriorate more quickly.
- It is usually better to change a smaller amount more frequently, rather than change a very large
amount less often. This minimises changes in the water chemistry and hence reduces stress on fish.
Some species are sensitive to large scale water changes: an example is Tanganyikan cichlids, where a
more sensible regime might be 15-20% every week.
- The nitrate level in a tank is often a useful indicator of water quality and the need for water
changes. The sensitivity of different species to nitrate and effects on growth and breeding are still
largely unknown. However, it is usual to maintain levels below 50 mg/l (=ppm), below 25 mg/l is better.
Specific resins or anaerobic filter systems can be used to reduce the level of nitrates, but
remember that nitrate is not the only substance that will build up. Regular water changes will
reduce the level of all potentially harmful substances, as well as replenishing minerals.
- Remember that the larger the percentage water change carried out, the more important it is to match
the temperature and water chemistry of the new water to that in the tank. A particular danger here is when
alkaline water is added to a neutral or acidic tank. The rise in pH to alkaline values will mean that more
of the total ammonium compounds present will be in the toxic form of ammonia (NH3).
Larger water changes will also increase the importance of using a dechlorinator if tap water is used.
- Test your source water to determine its pH and hardness, and also levels of nitrate and phosphate.
If your water is particularly soft and acidic or hard and alkaline, you might want to consider keeping fish
suited to this water, unless you are fully prepared to invest the extra time and money in modifying the water
chemistry. If levels of nitrate and/or phosphate are particularly high, you may want to invest in either products
which specifically remove these substances, or a reverse osmosis (RO) or deionisation (DI) unit.
- Purified water from an RO or DI unit is considered almost essential for keeping marine fish, especially
in reef tanks. It can also benefit freshwater fish by supplying water free of contaminants. The types of
nutrients which encourage algae - nitrate, phosphate and silicate - should be reduced to very low levels.
The 'pure' water should be adjusted to match the water chemistry of the tank, commercial products are
available for this.
- The question is sometimes asked as to whether it is possible to do too many water changes. The answer to this
question really hinges on how closely matched the new water is. If it is well matched to the tank water, then the
answer is probably no - the more water changes the better. An example often quoted is the scenario of a fish living
in a flowing river, where the water is being constantly renewed. Large lakes also have a huge body of water to dilute
wastes, and a much lighter stock of fish per volume than normally found in an aquarium.
However, if the water is not well matched, then the constant swings in temperature, pH, etc caused by
frequent large changes are likely to do more harm than good. This is why frequent small changes are ideal.
- When doing water changes, it is useful to vacuum the substrate at the same time, to remove
Below is a summary table of recommendations for water changes, remember these are only for general guidance - you
should test water regularly and observe fish behavior to ascertain that your maintenance program is sufficient.
Summary: Recommended Water Changes
|Minimum water change
|Recommended water change
|'Average' community tank (not fully stocked)
|25% per month
|20% every two weeks
|Well stocked community tank
|25% every two weeks
|20% every week
|Malawi cichlid tank
|25% every two weeks
|25-35% every 1-2 weeks
|Tanganyikan cichlid tank
|15-20% every two weeks
|10-20% every week
|Large cichlids and predators
|25-30% every two weeks
|25-30% every week
|Fry rearing tank (up to 1 month old)
|10-20% every 2-3 days
|10% per day
|Marine reef tank
|10-15% every month
|10% every 2 weeks