Ah water changes. A very important concept to understand and many do not see the importance of this.
First of all let me say the first few people that chimed in and recommended 50% once a week are very wise to do so. That is the very same advice I would give to any newcomer to the hobby or someone unsure of frequency. We can never do too many water changes. That said there are many, many factors affecting water quality so a hard and fast rule is difficult.
First of all let me begin by saying water quality cannot only be measured by the nitrogen cycle. In other words, Ammonia, Nitrite, and Nitrate are only a small part of the picture. The best way to measure water quality is to measure Conductivity. Conductivity is a measure of the total ions in water. This includes both total dissolved solids and pollutants such as nutrients (ammonium-nitrogen, nitrate-nitrogen and phosphate from fertilizers) and pesticides (insecticides and herbicides mostly) and various organic compounds. Henceforth we will refer to these organic waste compounds as DOC or dissolved organic compounds. We have to keep in mind that the nitrogen cycle deals only with proteins and there is far more in the water then bacteria breaking down proteins! Let me illustrate how short minded it is to measure water quality with just measuring for Nitrogen. If we measured the water of the Rio Orinoco we would find that it has zero Ammonia, Zero Nitrite, and Zero Nitrate. In addition it would have a conductivity of less then 20 us/cm.
In contrast if we have an aquarium, lets use one of my 50 gallon Angel fish fry grow out tanks as an example, with the same water filtered with a giant 20 gallon wet dry filter. We may have zero Ammonia, zero Nitrite yet the conductivity will rise dramatically. In just 24 hours we will see a jump from 20 us/cm to over 100. This is from all of the organic waste being produced buy the fish, bacteria etc. In contrast in nature the conductivity of a large stable system like the Rio Orinoco stays the same. If we measure today 20 us/cm we will measure the same tomorrow. The rainfall, the plants, and the natural bacteria cycles both aerobic and anaerobic breakdown all the organic.
So what does this all mean? Do as many water changes as you can!!! . In nature fish have very clean water with very little bacteria and organic waste. An aquarium no matter what type of filtration it has will always accumulate organic waste and this can only be removed by water changes.
Now there is one exception and that is in heavily planted tanks. Plants consume Ammonium as food. That is ammonia before it enters the nitrogen cycle. So this means we will never get nitrates because the plants are using up all or most of the organic waste as food. I have done experiments with this and I was able to have a 55-gallon tank heavily planted with some large swords and Aponogetons and water sprite. No filtration what so ever no even an airstone and have it populated with over 100 full-grown Cardinal tetras, a couple prs of Killies and a few Apistos. The conductivity of this tank began at 90 uc/cm from the source water into the tank and actually went below what we started. The plants were scrubbing the tank clean.
So what have we learned? Get a conductivity meter if you’re a serious hobbyist! You can get one reasonable online. You don’t have to spend $100 you can even get them for under $50 these days. With a conductivity meter you can see when you need to do a water change no more guessing. You will learn all the factors that truly affect water quality, food, stocking levels, fertilizers (for you plant folks) etc.
One last note many sensitive species that are hard to breed will benefit greatly from a stable conductivity. A stable conductivity is best achieved with water changes and plants! Sorry if this was too long.
I wrote some more on this topic on my website
Water Quality and Breeding
Breeding West african Cichlids
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