Do change 30-50% of the aquarium’s water every 1-2 weeks. This is variable based on fish load and filtration, but not on the aquarium’s size. From 50 Ltrs to 200 Ltrs, 30-50% is a good rule of thumb. This removes all the toxins and excess nutrients that build up in the water.
Don’t rely on filters to clean the water. Filters don’t remove excess nutrients and toxins from the water, they just trap it so it can be removed easier when you clean your filter. However, filters also can’t remove many things effectively.
Don’t change the water while cleaning the filter and viceversa…You can rinse the filter media in old tank water to remove waste build up. The nasty brown/green slime on the media is good bacteria that you don’t want to throw out or kill with chlorinated water. Rinse the filter media only once a month. Anymore could disrupt the beneficial bacteria and throw your tank into a mini cycle. When they start falling apart you need to leave the old filter in and run it with the new one for a few weeks to transfer the bacteria. Removing and replacing a filter without doing that will throw your nitrogen cycle away. A lot of your good bacteria lives in the filter media. If you take it out, you have les Bactria to deal with your ammonia and you get an ammonia spike.
Do make sure the water you use to fill the aquarium back up is the same temperature as the aquarium water. Drastic changes in temperature stress the fish and can lead to disease and even instant death in extreme cases.
Don’t forget to add a water conditioner that removes chlorine and/or chloramines if you are using tap water. These can kill fish and even plants if not removed. Water conditioners also often remove heavy metals such as copper that may harm plants and fish.
Do water changes after courses of medication, after stirring up the substrate doing a re-scape, or after accidentally overdosing anything, including fertilizers. Water changes help to “reset” the aquarium by removing dissolved waste and chemicals. They are often a cure-all for many aquarium related problems, and rarely cause harm if done in moderation.
Don’t remove more than 50% of the water during a water change if you can help it. Sometimes, more than 50% is needed in extreme cases (such as ammonia spikes or overdoses), but this can put stress on the fish and the beneficial bacteria in the aquarium. It can lead to rapid changes in water parameters (such as pH) that are as stressful to fish as rapid temperature changes.