The final product of the Nitrogen Cycle is nitrate. This is a compound that is used by plants as food, and the plants convert it into oxygen. Even if you have a heavily planted aquarium, nitrates will build up faster than most plants can use it. This will require regular water changes.
If you asked 10 aquarists what percentage water change they do every week, you’d get 10 different answers. It all boils down to tank stocking, type of fish, and personal experience. Some people advocate big water changes, while some do 10%-20% each week. I’m in the former category.
My current aquarium (generally) gets a 50% water change each week. This allows me to vacuum out the substrate thoroughly as well as do any maintenance without having my arms shoulder deep in fish water.
If you only have one aquarium, this process is an easy one. Get your new water into a container using only the cold water tap* and adding your pH buffer and water conditioner.
*For those that don’t have mixer taps, the hot water tap can contain more contaminants and heavy metals.
I personally use a Python No Spill Clean and Fill for my water changes. It allows you to hook up to (some) faucets in your house, but you can also use your outside water spigot like I do. The same hose can perform the fill and empty procedure, but you’ll want to fill buckets so you can treat your water before adding it to your aquarium.
The attachment at the end of the hose allows you to clean your substrate. The suction is weak enough that it doesn’t suck up any of your substrate or your fish (if you’re careful). Once the substrate is clean, do any extra cleaning you desire (filter spray bar, filter intake, heater, rocks/ornaments, etc.) and fill the tank back up. I recommend using a bowl or plate on the surface of the water to prevent the inrush of water messing up your aquascape.
This should be done weekly to ensure optimal water quality. Once the water is changed, wait about an hour and test your water parameters. If any of your parameters (ammonia, nitrite, or nitrate) are still too high, you may want to do another 50% water change the following day, and repeat this until your parameters are below toxic levels (ideally close to/at 0 ppm). Multiple water changes per day can have ill effects on your fish since they need time to adjust. Some products can detoxify ammonia and nitrite; however, I discourage the use of chemicals such as these in the aquarium and believe that water changes are a better solution. If you do go this route, be sure to follow the directions exactly.
If you have multiple aquariums, I advise making a schedule based on what you prefer to do. If you want to do all the maintenance of your tanks on the same day, pick a day you’re free and go for it. Or you could spread out the maintenance over a few days and focus on one or a few tanks at a time.