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Why Make Partial Water Changes?

It is often stressed that making regular partial water changes is absolutely essential for the well being of the fish and plants. At its most basic level water changes will freshen up the environment for the fish and plants – imagine living in a small room with no ventilation and having to live there 24 hrs per day. This includes toileting, washing and eating and then once per month someone came and opened a window for a short time. Most of us would not like this but that is how it is in a fish tank should it be neglected in any way.

aquarium water change

Citron / CC-BY-SA-3.0

What do partial water changes actually do?

Because of all the biological processes going on in an aquarium, various changes take place in and to the water. Most commonly, nitrates build up because this is the end product of biological filtration in the majority of aquariums. But there are other things going on in the environment too. Biologically important minor and trace elements get used up, carbonates get used up and, if left unchecked for long enough, the pH will begin to fall. The total dissolved solids will rise, pheromones from the fish may rise and there will be a rise in dissolved organic compounds. None of these changes are desirable or good. Regular partial water changes if carried out often enough and with a large enough volume will keep everything in check and create a much better environment for the fish and plants.

Old tank syndrome

Old tank syndrome occurs when insufficient water changes have been made. If you stick to the often quoted formula of changing 25% of the water every two weeks you may find that the nitrate level is slowly increasing, the pH is falling and the water becoming ever harder. This goes unnoticed at first but it will eventually impact on the fishes health and that’s when you discover that you have a problem even though you have carried out the partial water changes as directed. It is essential that nitrate and KH are monitored especially in the early days of a new aquarium and that the results of those tests are used to formulate and fine tune the volume and frequency of water changes in order to keep things in balance.

Over time, décor elements such as bogwood and rocks can alter water parameters. Organic materials tend to lower the pH, and release tannins resulting in ever darker water. Rocks on the other hand tend to increase the pH, especially calcareous ones, which makes them an ideal buffering material for cichlid tanks, and a nightmare for soft water fish. Failing to change water regularly will allow these materials to have a more substantial impact on water parameters, often with serious consequences for livestock.

How much and how often?

The formula answer is 25% every two weeks. But this advice is worthless in reality because every tank is different and as such each tank needs its own maintenance schedule. If insufficient water changes either in frequency or by volume are carried out then the tank will slowly decline. You can test the water for nitrates and carbonate hardness which means that you can use these measurements as a guide.

You should aim to keep nitrates below 25mg/l (ppm – parts per million) the lower the better. If your nitrate level has risen from 12mg/l to 25mg/l and you carry out a weekly water change you need to change 50% of the water using nitrate free water as a replacement in order to keep things stable long term. Normally there will be sufficient carbonates in the replacement water to keep the carbonate level stable unless you live in an area where the water is particularly soft, in which case you will need to boost them using an additive before adding the water to the tank. If you are unlucky enough to have high nitrates in your supply water you can use rain water or r/o water (reverse osmosis water) both are nitrate free. You will however need to re mineralise both to the desired level before use in the aquarium.

About Andy Rapson

I've been interested in fish for about fifty years and I have kept many different species in that time. I have also worked in the fish trade running my own fish shop and I'm a Fishbase collaborator. I'm now mainly interested in fish husbandry, fish health, native marines and fish photography.

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