Advice on Changing Water in Your Aquarium

Water changes are important even if the water looks crystal clear. Read on to find out why.

Why do we need to perform water changes?

In the wild fish are treated to constant water changes in the form of rainfall. They generally live in vast bodies of water with far lower stock density than the average home aquarium and they have the added advantage of nature’s filter, (plants, algae and anaerobic bacteria (more on that later)).

On the other hand, our fish live in a closed system where contaminants build up and up with no other means of escape other than manual water changes.

One of the main contaminants that we should be keeping an eye on is Nitrate. We spoke in a previous blog about the nitrogen cycle and how Nitrate is the end product. In low concentrations it is harmless, however, if it’s allowed to build up it will become hazardous for fish and promotes the growth of unsightly algae.

Fresh water also contains minerals which are used by fish, plants and bacteria. Some of these minerals are also required to keep PH levels stable. Old tank syndrome is a term used to describe a well-established tank that suddenly “crashes”, often resulting in mass casualties. This is due to the system running out of essential minerals causing the PH to drop, bacteria to die and ammonia to rise.

It’s also worth mentioning that fish release hormones into the water. Some of these are thought to stunt growth and reduce breeding activities. Water changes serve to dilute these hormones.

How often should we change water and how much to change?

It’s always best to keep things stable within an aquarium. This includes how much food we feed, hours per day that the lights are on and how much water we change.

Firstly, I recommend testing your tap water for Nitrate. Many areas of the UK suffer from high nitrate levels in our tap water. This will severely affect the percentage of the water that we need to change. I’ll be writing a blog in the coming weeks on how I deal with nitrate levels of around 40PPM in my tap water.

Next, we need to decide how often we would like to perform water changes. Personally, I prefer weekly water changes, others find monthly water changes more practical. The most important thing is to be consistent. For this example, I’ll work on the basis that weekly water changes will be performed.

To determine the volume of water to be changed, we need to get an idea how much waste is produced over the course of the week. We can do this by testing nitrate levels after a water change and again just before the next one is due. Ideally, it’s best to keep nitrate levels below 30ppm in a planted tank and below 20ppm if the aquarium is not planted. This of course is made much more difficult if your tap water already contains nitrate and may require other measures which we will touch on later.

If our aquarium is producing 15ppm nitrate in a week and we perform 50% water changes weekly then we can expect 30ppm nitrate just before a water change and 15ppm just after. It may take a while to determine the levels that your aquarium balances out at and you may need to increase or reduce frequency of water changes and quantity of water changed to suite your particular setup. Once you have fine-tuned your routine, you can test a little less frequently but I’d still recommend testing at least once per month to make sure that things are still on track. This is particularly important if you increase or reduce stock or the amount of food fed.

We mentioned earlier that water changes also replenish minerals. It’s therefore advisable to test GH KH and PH. It’s not essential to target a specific level of any of these parameters, it’s more a matter of making sure they don’t change too much. If GH or KH begin to drop, it can be a sign that larger or more frequent water changes are required.

So how do we change water?

In a small tank, I’d use a bucket and a gravel vacuum. Simply use the gravel vacuum as a siphon to suck water out of the aquarium into the bucket. Whilst removing the water, lightly vacuum the substrate to remove fish waste and uneaten food.

If your aquarium is large and you’re changing a large volume of water, I’d recommend using a submersible pump connected to a garden hose. You can still vacuum the gravel using the above method but the bulk of the water removal will be performed without carrying multiple buckets of water around.

To refill, you should match the temperature of the incoming water to the aquarium water as closely as possible. It’s not necessary to have it exact, just dipping your fingers in to make sure it feels a similar temperature is good enough.

For a small aquarium, I would use a bucket, fill it with temperature matched tap water, add dechlorinator and slowly pour it into the tank.

For larger aquariums, I recommend finding an adaptor to fit a mixer tap. Attach a hose to the tap, get the temperature set and fill directly from the tap. If using this method, you need to add dechlorinator to your aquarium as it fills. I tend to add enough dechlorinator for the whole tank. i.e. if I’m performing a 25% water change on a 400 litre tank then I’ll add enough dechlorinator for the full 400 litres.

There are some ways to reduce the quantity of water changes required.

Plants consume waste in the form of ammonia, nitrite and nitrate. This is how the waste is dealt with by nature and we can also benefit from this in our home aquariums.

Nitrate reducing resins can be very useful if keeping sensitive species such as Discus or to help keep things safe in a heavily stocked aquarium. However, these resins do need recharging regularly in a salt solution so they’re not a set and forget fix.

Anaerobic bacteria are another great method of reducing nitrate but it’s very difficult to achieve in a home aquarium. Unlike aerobic bacteria which converts ammonia to nitrate, anaerobic bacteria require very slow flow and very low oxygen. This can be achieved using a deep sand bed which must remain undisturbed. This particular strain of bacteria takes at least 6 months to become established.

The above methods can help to reduce nitrate but do nothing to add minerals or remove other toxins that can build up. Therefore, it’s still essential to perform regular water changes.

Dealing with nitrate in tap water

Click here to read our blog dedicated to this topic.


I hope you found this information useful. If you have any further comments then please leave them below. You can also contact us for free help and advice.

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