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Step 1
Ensure your tank is stabilized.
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Step 2
Decide how large you want your fish to be. Freshwater aggressives can be small (like a two inch male Betta), medium (six inch Convict), or large (twelve inch Pacu). Narrow down the choices by eliminating fish too large for your tank.
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Step 3
Know your size ratio. For small species, size ratio is generally one inch of fish length to one gallon of aquarium water (1:1). However, with larger species such as aggressive cichlids, the water portion of the ratio increases dramatically (1:5). For example, a ten inch Oscar requires a 50 gallon long tank. When in doubt, buy fish that are smaller in size. Never overcrowd a tank.
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Step 4
Understand the difference between tall and long when determining how many fish to buy. Not all tanks are equal. Most aggressive aquarium fish swim primarily side to side in the tank, rather than top to bottom. That means a 40L (40 gallon long rectangle) tank has more swimming area than a hexagonal 40T (tall). Keep this in mind when determining size ratio.
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Step 5
Choose fish of similar (lifetime) size and temperament. If you're new to aggressives, start with cichlids. They're among the most hearty available. Ask how large the species will grow. Convicts and Oscars are both cichlids, but Oscars normally outgrow Convicts by a great deal.
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Step 6
Buy fish living in the same aquarium if you can. They've already developed a hierarchy and are less likely to fight than fish originating from separate tanks.
