To keep the fish healthy, it is important that you keep their living conditions in as best order as possible.

HEATING

The heating system for the modern aquarium is entirely reliable and automatic. The thermostat is set to a narrow width of heat range and merely switches the heater 'on' and 'off' as required.
Small aquariums will only need a small heater/'stat, up to 110 watts maximum, whilst the standard 60cm tank can operate quite adequately with a 150 watt unit if the aquarium is kept under usual indoor temperature conditions.
In large tanks, split the larger heating load required into two separate heater/'stat units, placing a unit at each end of the tank.

FILTRATION

Filtration is all about keeping the water clean (note there is a subtle difference between 'clear' and 'clean') and is achieved by three methods.
Mechanical filtration physically strains out suspended debris in the water using filter 'wool.'
Chemical filtration uses materials such as activated carbon to adsorb dissolved materials in the water.
Biological filtration, probably the most important aspect, uses bacteria to render toxic ammonia and nitrites into less dangerous nitrates which may be used by plants as nutrition.

Power filters (either internal or external) are mainly mechanical and chemical filters although they may act biologically once fully established and over-dirty!
Biological filters may take the form of internal, undergravel filters or external fluidised bed filters.
Protein skimmers, as found in marine aquariums, do not work in freshwater.

All means of filtration, regardless of method, also bring some degree of water circulation to the aquarium which not only agitates the surface - assisting oxygenation and expellation of carbon dioxide - but also helps to dispel any 'cold spots'.