Water quality in Aquariums
The water quality inside the aquarium is important to all aquatic life and deteriorates for many reasons.
One of the biggest factors in deteriorating water quality is fish waste. Fish produce waste and this can accumulate in the water over time. Dead fish and uneaten food also add waste products to the aquarium water.
All of these waste products will become toxic to the fish over a very short period of time. A mature tank will be able to deal with some of the waste through beneficial bacteria that will actively feed off the waste products. Plants will also use the waste products to their advantage. However you will need to perform regular water changes to physically remove most of the built up waste.
We are lucky nowadays because this job has been made easier through advances in technology. External canister filters are very good at removing some of the waste from the water and collect the waste for easy disposal when filters are cleaned. This does not mean that water changes are not needed, they are. A filter will only slow down the rate of how quickly the aquarium levels of waste build up.
Because waste created by fish is the largest problem for a hobbyist, then a simple solution is to make sure you don't over stock your aquarium with fish. It is much better to lightly stock an aquarium and in doing so the aquarium ecosystem will be much happier.
Make sure you have no dead or decaying fish in the water as this will quickly pollute the water.
Because feeding fish in the aquarium is so enjoyable, many people end up over feeding the fish. This is a big problem because over fed fish just like people become unhealthy, plus the excess food just sits in the aquarium and starts to pollute the water as is starts to decay.