A general rule of thumb is one inch of adult fish per net gallon of water, but activity level, territoriality, available habitat and filtration should also be taken into account. Remember, the average aquarium holds less water than its designated size once gravel and decorations are added. (For example, a fully decorated 10 gallon aquarium holds about 8 gallons of actual water). In addition, most fish that are purchased are going to grow, and some fish just need more personal space, so plan ahead. Always consult an aquarium expert when adding new fish to your aquarium.