Water changes replace minerals, which help to 'buffer' the water and maintain stable conditions, and reduce Nitrates (NO3) that may gradually build up over time, causing long-term damage to your fish
No matter which book you read, which shop you ask, or what your fellow fish keepers say, you will always be recommended different amounts for water changes. Working out who is right is impossible; the correct amount of water changes for your aquarium depends on a number of factors that need to be taken into account.
To create a regime of regular water changes for your tank it is important to understand exactly why we change certain amounts of water and not others. Getting it wrong can result in disaster, and this applies to both too little and too much.
Water changes to dilute nitrates
If you do not remove and replace enough water on a regular basis, levels of nitrates, which are produced as a by-product of filtration, will steadily rise. Unless you are monitoring your nitrate levels, you are unlikely to notice this.
The fish in your aquarium will adjust easily to the gradual rise in nitrates and will only experience harm over a long period of high levels, although they may become more prone to diseases.
Ideally, your nitrate levels should be around 50ppm or less, once your levels reach up to 200ppm or more, you might have a problem. Any new fish which you introduce to your aquarium are likely to be used to low levels of nitrates, if they are suddenly exposed to high levels in your aquarium, they will be placed under considerable stress and may even die. In serious cases, the death of a new fish causes fluctuations in the aquariums water chemistry balance, which causes some existing fish (already weakened by the high nitrates) to also become ill, and possibly die. A vicious cycle is then created which can result in very high fish losses.
Most good retailers are well aware of this problem, as they often get the blame for the fish deaths. From the fish keepers point of view, they have bought some new fish, placed them into their tank (which has had no problems at all) and shortly after, the new fish have died, along with several of the existing fish. In this scenario you can see why the fish keeper would immediately blame the new fish and/or the shop, but the truth is that it was caused by bad maintenance on the fish keepers behalf.
By regularly changing some of the aquariums water, the nitrate build-up is constantly diluted and kept at a low level. In some situations (e.g. high nitrates in tap water) it may be useful to also use a nitrate removing media as part of the aquariums filtration.
Water changes to replace minerals
The second problem with too little water replacement is the loss of minerals in the aquariums water. Minerals help to 'buffer' the water, or keep it stable from fluctuations in pH (acidity/alkalinity) Without getting too technical, this works as follows; In an aquarium acids are constantly produced, these acids are countered by alkaline minerals (buffers) so the pH level remains stable. If those mineral buffers were to become used up, any further acids produced would dramatically affect the pH level, which would drop, and the water would become more acidic.
Once the buffering capacity of the aquarium has dropped to virtually nothing, a 'pH crash' can occur, where the pH drops by several points in a short period of time. If a severe pH crash occurs, it can be enough to wipe out an entire tank of fish. Obviously, this is an unwelcome situation. Changing water regularly replaces minerals that have been used up, thereby maintaining a constant buffer, preventing pH crashes.
Too much of a good thing
Now that is has been established that water changes are vital to the health of the aquarium it is important to understand what can happen when too much, rather than too little, water is changed. Although a water change is generally beneficial, it is still a disturbance to the aquarium environment.
Some fish find this a welcome thing, and a water change can even trigger spawning in some fish. Too much water being changed at once however, will harm the majority of fish. If for instance, around 30% or more of the aquariums water was removed and replaced in one go, the resulting fluctuations caused by the difference in water parameters and temperature between the remaining tank water and the new water, would be severe enough to cause harm. Such a large change, and a resultant large fluctuation, can cause shock to a fishes body, damage filter bacteria resulting in water quality problems, and harm delicate plants.
Another problem also occurs when too much water is changed, although this does depend on the quality of water you are using. In most cases, fish keepers use tap water as their source, which often contains a high amount of minerals, nutrients, and chemicals (de-chlorinator or tap water 'conditioners' do not remove these) These various substances are usually present in excessive amounts and will cause algal blooms and toxic build ups in fishes bodies.
You can use this water in an aquarium because most of these toxins are taken up by plants, or bind to organic (waste) matter and settle in the substrate, out of harms way. It is during this period of 'settling down' in a newly filled aquarium when algal blooms are likely and fish losses common (although other factors are involved)
Each time you do a small water change, you are only introducing a diluted proportion of these substances, so the effect is minimal and the substances are soon taken out of harms way. If however, you continually carry out large water changes, these substances may always be present at harmful levels, causing an overall imbalance and ill health in the aquarium.
Frequency verses volume
An aquarium should be a stable environment, so any major changes are detrimental and should be avoided unless done in an emergency situation. Therefore it is always best to do smaller water changes more frequently rather than large water changes less frequently. A 10% water change twice a week is much better than a 20% water change once a week.
If you have an open topped aquarium, you may find that you lose a lot of water through evaporation. When water evaporates, it is only pure water that is lost; everything else contained within that water is left behind, so the remaining water becomes more concentrated with substances. This means that if you continually top up your aquarium, you are continually increasing the concentration of some substances in the water. Topping up is therefore, not equivalent to carrying out a water change and should not be considered as such.
Tap water is usually safe to use for most fish, providing it is treated with a de-chlorinator to remove both chlorine and chloramines. The quality of tap water varies around the country, but it is always best to assume that your water is of low quality when it comes to your fish. Therefore you should always try to do the minimal amount of water change needed for your aquarium, or alternatively, use a good chemical filtration medium in your filter to remove the tap water 'nasties'.
Of equal importance is the hardness of your water; hard water is generally high in buffering minerals, so providing your nitrate levels remain low, you can do less water changes. If your tap water is very soft, you could do more water changes, or alternatively, add an 'off-the-shelf' aquarium buffer/mineral additive.
If you use R.O. water, you should either mix it with at least a third tap water to introduce some hardness, or if you want the best possible quality, use a mineral/buffering additive.
The plants factor
Live plants have many benefits for the aquarium and amongst these is a tendency to remove nitrates from the water and the production of 'base compounds', which stabilise pH levels.
An aquarium that is heavily planted therefore requires less volume of water to be changed. A 'true' planted tank has over 80% of its area covered by plants, but for this article and the following flow diagram, we can consider 40% or more as heavily planted.
Do not assume however, that if your tank has high nitrate levels, the answer is to add lots of live plants - this will not always work! Although plants do use up nitrates, they can only do this is small amounts and high levels can be detrimental. Plants must first be growing healthily in low nitrate conditions before they will start to take up significant levels of any further nitrates produced.
Working out your water changes
Finally, you can now work out the amount of water changes your aquarium needs. Your hardness level dictates the amount you need to change to maintain buffering capacity (soft water = more frequent) and your stocking density and number of plants is a good indicator of the amount of nitrates produced (less plants + high stocking level = more nitrates)
The flow chart below will give you a recommended level of water change based on these factors. If you have large, messy fish, or tend to feed larger volumes of food, then count your aquarium as having a high stocking level.
For any aquarium, it would be best to change at least 20% each month (preferably not all at once, e.g. do two 10% changes per month) and no more than 20% per week. If you still have high nitrate readings, even when doing a 20% change each week, you should use a dedicated nitrate remover rather than increase the water changes. If your source water (e.g. tapwater) already has a nitrate level of over 50ppm, you will certainly need to use a nitrate remover in the aquarium, and carrying out more water changes may do little, nothing, or even raise tank nitrate levels.
In the flow chart below, soft water aquariums require higher volumes of water change; these can be reduced if you are using a mineral additive/buffer in your aquarium.