Water Changes – Why Are They Performed

Part of all aquarists maintenance routine should be performing a water change on the aquarium. Water changes are not just limited to saltwater aquarium – all home aquariums have a requirement for their water to be changed.

I was doing my water change today as part of my weekly maintenance regime and I thought to myself ‘how many people change the water without actually knowing why they are doing it…’

This article is aimed at owners of saltwater aquariums as there are slight differences between saltwater aquariums and other home aquariums – one of which is salt of course!

The recommended amount of water to be changed is 10% per week however this is not a figure which is set in stone as more may be changed if there are problems with the aquarium and less may be changed for various other reasons.

Due to the percentage water change this does mean that some people will be changing a small amount of water and others will be changing a lot more. Personally I have an aquarium where I am able to make up enough reverse osmosis water to perform the change. My aquarium is 100 gallons so 10 gallons is relatively easy to produce as my reverse osmosis unit is quite quick.

Other people decide to make up a large batch of reverse osmosis water and keep it for use until the container runs dry.

It’s whatever works best for you really.

If you look at the back of the box/container which the dry salt mix comes in you will see that it is made up of many things. This is one of the reasons why we change water.

The animals which we keep in the aquarium, especially corals, use some of what is in the water. Each of them has a different demand and slowly but surely the amount in the water is slowly reduced. When a water change is performed this element is added back to the water so effectively the element(s) being used are topped up which the livestock can then use.

One of these such elements is calcium. A lot of life in aquariums require calcium. When a water change is performed calcium is added back into the aquarium. For some aquarists this will be enough to maintain the calcium at the required level however for others they may need to use alternative methods to top up the calcium levels.

The replacement of required elements is a very important reason to change the water in the home aquarium however it is not the only reason.

Another reason we change water is that we are able to remove dissolved organic compounds from the aquarium before the filtration has had time to break them down. The protein skimmer attached to the aquarium can only process the water which flows through it and not all of the water in the aquarium will do so. It is recommended that when you perform a water change that you clean the rockwork as well. By blowing the detritus etc on the rock work in the water column this can be removed at the same time as the water. When a water change is performed some of the dissolved organic compounds are also removed and the water which replaces it is nice and clean.

If a sand bed is used for decorative purposes only i.e. it is not a deep sand bed or a plenum then the time when a water change is being performed is a perfect time for cleaning the sand. Stirring it is fine as all the detritus etc will flow up into the water column and can be siphoned out with the water.

When water is removed from the aquarium any nutrients in the aquarium are also removed. One of these is the dreaded nitrate. All aquarists aim for readings of zero when it comes to nitrate however water changes can be used to control nitrate. For example if you have 10ppm for nitrate in the aquarium and replace 10% of the water then your new reading should be 9ppm. Over time and with careful management this nitrate reading can be reduced further. Another nutrient which can be controlled/managed via water changes is phosphate.

Therefore effectively the main reasons we perform water changes is to add essential elements back into the water which have been used by corals, fish etc and also to remove nutrients, detritus etc from the water whereas if left they could be broken down by the filtration and become problematic.

I personally belive that water changes are one of the first things to be skipped when aquarists become lazy with the aquarium care and maintenance. At the end of the day we perform water changes so that we can attempt to achieve what all aquarists attempt to get – excellent water quality. If water changes were not performed then the fish, corals etc which live in this confined environment will be living in an environment which is not suitable for them.

Would you like to live in an environment which was polluted, full of nutrients and over time would make you poorly? I know I wouldn’t – so why make your livestock.


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1 Comment
  1. Good article, another way to look at this is we perform water changes to remove waste. Waste doesn’t evaporate with the water, it’s left behind like salt.

    So performing a water change is like flushing the toilet. So how often do you flush your aquarium’s toilet?

    Another reason is to replenish essential trace elements, but it should not be to replenish calcium and alkalinity. These should generally be maintained at a level higher than dissolved salt mix. To achieve these elevated levels you will need to dose with calcium, alkalinity and magnesium.

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