You say caring for the water is more important than caring for the fish ?
In a the natural environment, tropical fish often live in a huge volume of water such as a river or lake.
If the sun shines, it will take days to alter the average temperature. Fish and plant waste may decompose producing ammonia and nitrogen but it will be so heavily diluted it couln't possibly hurt the fish. Even the worst disaster is very heavily diluted.
In the aquarium the opposite is true. The tiniest change affect a small volume of water really quickly.
Add to this the fact that our humble tapwater has stuff added which is designed to stop things living in it. This means that we need to really care for our water if we want fish to survive.
The first thing to consider is the tapwater you'll add when you first fill the tank, and in subsequent water changes. Tap water contains chlorine in many areas, and chloramine in others. Chlorine is a gas which will naturally escape if water is left to stand. When you first complete a tank water fill, just leave the water alone for a couple of days and the chlorine with have dispersed. When doing water changes, either leave the water to stand or add a neutralising liquid (For reasons of volume, it's better to use a neutraling liquid). In some water areas, the tap water is steralised using chloramine. This is harmful to fish, plants, and friendly bacteria. Cholamine is fixed into the water supply and will not be removed by leaving water to stand. A tap water neutralising liquid has to be added to tapwater containing chloramine. To find out more about your tap water visit your local water company website. Most have an interactive feature where you input your postcode to see the latest chemical analysis of your tapwater.
In-filter heater (where the heater is hidden in the filter housing, like in Juwel aquaria)