When planning your beginning aquarium stocking strategy, assess fish size, hardiness and aggressiveness.
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Size
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One of the first things to consider is the size of your aquarium. Aquariums have a livestock-to-water ratio that promotes optimum health. A good rule of thumb is 4 inches of small-to-medium fish for every 10 gallons, or 2 inches of larger or faster-growing fish for every 10 gallons. If you stick with smaller fish such as blennies or a gobies, you will be able to stock more fish with a larger variety, and still meet the water quality requirements. Clownfish are also very common in small beginning tanks because they have nice coloration, are very hardy and can live for many years.
Hardiness
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Your tank's maturity is a factor in the type of beginning fish you select. For example, if you have just finished cycling the tank, it is better to start out with a very hardy fish that can overcome small mistakes or additional imbalances that might occur. Damsels are a great beginner fish because they are attractive, aren't picky eaters and can survive in less than pristine water conditions. The drawback to damsels is that they can be aggressive, especially once they become established. You can mitigate their aggressiveness by only keeping one, but you will need to watch it carefully when you add other fish and remove it if needed.
Aggressiveness
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Many fish are territorial in different situations, such as towards specific species, body shapes or specific quantities. The dottyback, while generally a good hardy fish for a beginning aquarist, can be aggressive. According to Aquatic Designs in Plano, Texas, the dottyback is primarily aggressive towards fish with the same body shape, but is not necessarily aggressive toward other types of fish. As a general rule of thumb, the more established a fish is, the more aggressive it is toward new fish.
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