The thing to remember is to distribute the food evenly throughout the water surface. Avoid spot feeding. The rule of thumb is about 1 inch of fish per 2 Gallons of water. This guideline should only be used in order to get a sense of the limitations of your tank. However, some species can be kept in a 10-Gallon tank, others need more physical space.
Territorial fish require a larger tank as they chase intruders out of their claimed territory.
Fluctuations of temperature should be moderate and controlled at all times. The optimum temperature varies from fish to fish, but too cold or to warm will cause stress. Keep in mind that a tank in direct sunlight will heat up during the day. The water temperature may also rise during the light-on period. This can be more of a concern with smaller tanks.