The role of water changes in closed systems is fundamental to the long-term health of animals and plants. The length of time in between water changes is highly variable. The stocking levels, water chemistry, metal toxicity, excessive nutrients (most common - N, P, Fe), and allelochemicals should all be factored in when determining a water change schedule for your systems.
Aquariums and ponds have nutrients added daily with fish food. Plants do not use the added nutrients at the same ratios as it is added. Without water changes, some nutrients can reach toxic levels to plants and animals.
Topping off systems due to evaporation can affect the water chemistry over time. If tap water is used for topping off a system, it can increase the GH, KH, TDS, and heavy metals depending on the chemistry. If tap water is very soft, it may not add enough KH to keep up with plant and bacteria consumption. If distilled or RO water is used, KH will decline over time, causing the system to become increasingly acidic.
Water changes help reset concentrations of chemicals in the solution. Water changes are also a first step hobbyists should take when they suspect the health of the aminals or plants is in decline. Accidental introduction of toxins can happen, and a nearly 100% water change can often be what the system needs. The solution to pollution is dilution.
A large percentage (> 90%) of water change is acceptable as long as the water chemistry (GH, TDS, KH) is similar. If the system needs additional minerals, add the diluted minerals to a high-flow area after a water change.
The new water's temperature is an issue often overly debated by novice fish keepers. Advanced fish breeders often use much cooler new water ( > 10°F, 5.5°C) to stimulate fish to spawn. In nature, fish swim through thermoclines daily, where the temperature can change significantly in a second. Heavy rainfall also changes the temperature of natural aquatic systems within minutes. Thermoclines can also develop within the aquarium systems.
Considering all factors that can affect the water quality in a system should help determine the water change schedule. Water changes should be logged to help determine if the schedule is adequate or needs adjusting (frequency/percentage).