An aquarium is essentially an entire world. If it is large enough, and made ecologically stable, you might be able to get away with almost zero maintenance. Having some herbivorous inhabitants to eat plants, some carnivorous fish to eat the herbivorous if and when they breed, the waste from the inhabitants to feed the plants, and then just add light (the sun). Unfortunately, in my experience, that is not possible.
Inevitably, we over-stock the tank resulting in pollution that need to be removed. We feed the fish - introducing pollution.
So, it is essential that we have systems put in place to support the life we put into the tank. Critical to aquatic life is water - good, clean water. I briefly mentioned a canister filter in my earlier post about biological filtration and cycling the water. The canister filter which provides biological filtration (which removes the pollution caused by life) and mechanical filtration (the "dust and dirt") is an essential part of a life support system that we put in place to make this world we have put together inhabitable.
Here's a picture I took at the aquarium I visited in Toronto - while that Life Support System is many times the size of the one for my tank, essentially the same elements are there.
The canister filter removes pollutants and dirt but more is needed. As a rule of thumb, the canister filter should cycle the water at least 3 times per hour. More is better, there is no such thing as over-filtration, under-filter and pollutants build up. In a well set-up planted tank, the plants, rocks, driftwood and substrate will also colonize bacteria and act as biological filters to remove pollutants, but they should not be relied on as the sole method of filtration. Different fish also pose a different pollutant load - that will be considered later.
Pollutant build-up cannot be removed with just filtration alone, and it is essential to change part of the water every week or so. Do not just top up the tank replenishing water lost due to evaporation as this concentrates pollutants. I recommend changing 20% of the water every week. Change too much at a time and you tend to de-stabilize the eco-system. Change too little and pollutants build up over time.
There are also a couple of schools of thought about the water. There is one school who would set up a RO (reverse osmosis) system so that water changes would be with the purest water possible and add the necessary chemicals to sustain life. However, I do feel that the micro-nutrients (impurities) in spring/natural water is essential to life. This is another subjective/objective debate that I won't go into. It is the chlorine/chloramine/fluoride that has been added to tap water that is supposed to be bad for fish and plant life. However, I've been successful so far just changing the water using tap water and adding Seachem Prime to remove chlorine/chloramine/fluoride. As I mentioned earlier, Seattle has great quality water. However, every now and then, I would also try to add natural, alive water to the tank.
Find a spring, bring a few clean containers and harvest some water for your tank.
Unless you have some special needs like extremely soft water for discus or alkaline hard water for Rift Valley cichlids, tap water plus a little conditioner might be sufficient.
Besides clean water, it needs to be at the correct temperature for the fish you intend to keep. Discus, for example, require very warm water. Plants thrive in generally cooler temperatures. And fish do not like rapid temperature changes much. If we are keeping tropical fish, a heater is usually necessary unless you are living in the tropics. Sometimes, when you are living in the tropics, you need a chiller as room temperature may be too warm. I like to use an in-line water heater that is part of the life support system instead of an in-tank heater. Fish are cold-blooded animals, and I have seen fish snuggle up to an in-tank heater killed when the heater kicks on and boil the fish nearby.
This will attach to the outflow of the canister filter and will heat the water flowing into the tank. Unfortunately, the thermostats on most of these heaters are "estimates" only. So, invest in an accurate digital thermometer and adjust the temperature-setting dial ignoring the temperature printed on it. If you already have fish or plants in the tank, make changes SLOWLY. Take your time to get it to the right temperature.