Do not over feed. Fish do best when fed 5 days a week. A few days without food is good for their metabolism anyway. Furthermore a fully cycled tank offers a continuous supply of edible microscopic particles so fish pretty much can never starve.
Remove uneaten food. Never let flakes accumulate in the corners after a feeding, scoop out what they don’t eat. Food rots very quickly in the water which causes ammonia spikes, the number 1 killer of fish.
Never remove more then 25% of the water during a water change. Removing more will disturb the natural water chemistry balance which stresses fish. An exception is when medicating the aquarium, directions on the package should be followed.
Never empty and scrub out the tank or clean gravel and ornaments with soap. This will kill beneficial bacteria disrupting the natural cycle. If you are having algae problems on the ornaments, rub the algae off with your finger in the tank, or rub off in the bucket of water that you’ve just removed from a water change. Do not use tap water as the chlorine will kill the good bacteria.
Always use water conditioner when adding new water. Chlorine kills fish. An alternative to water conditioner is leaving the bucket exposed to air overnight, chlorine evaporates. Water conditioner is recommended however as there are usually elements within that maintain fish health.