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You must know how big your fish is likely to get and adjust the tank population accordingly. That cute little fellow may grow to six inches and regard its neighbors as a snack.

Select fish that are not only compatible with your tank but also with each other. Not all species mix well and yours won't thrive if they're being chased around all day.

Research thoroughly any species you 'd like to add to your tank and buy the number that ensures they'll interact as naturally as possible.

Safely adding fish to reach your tank's population limit can take six months or more. Be patient.

Too many fish in your tank makes it impossible to maintain good water quality, which adds to the inherent risk of your fish becoming sick and dying.

Tank Size
  • Freshwater: minimum 5 gallons plus, recommended 20 gallons plus
  • Saltwater: minimum 29 gallons plus, recommended 55 gallons plus
When calculating the number of fish your tank can hold, take into account the shape as well as its capacity. Water surface area is important. Gas exchange (primarily oxygen) takes place at the surface, and tanks with a larger surface area will be more efficient. A further advantage to a tank with a large surface is the ability to easily landscape the bottom. Think about using long, wide tanks instead of tall, thin ones.

Fish Load

You need to know what size fish population your tank can take. Here are some rough guidelines:

  • Freshwater: One inch of fish (excluding the tail) per gallon of water
  • Saltwater: One inch of fish (excluding the tail) per two gallons of water

These rules are not hard and fast and you'll find several variations. Your tank's fish population can be increased or decreased by the factors given below:

Positive Factors

  • Regular water changes
  • Live, healthy plants
  • More than one maintained filtration system
Negative Factors
  • Irregular water changes
  • Unhealthy live plants or no plants at all
  • Limited or badly maintained filtration system
In trying to create the ideal environment for your fish, you have to be aware of what they need to thrive. Not only do they have to be pleasing to your eye, you need to know:
  • How big is this species likely to get?
  • Is this species peaceful and compatible with most fish?
  • Is this species aggressive or semi-aggressive?
  • Is this species especially susceptible to stress?
  • Is this species a bottom swimmer, mid-level swimmer or upper-level swimmer?
  • Is this a schooling fish? If so, what's the optimum number for a school to interact successfully?
  • Is this species one that bears live young?
  • Sex your fish (if possible). Are they likely to breed?
  • Is this species one that can tolerate only its own kind, and are they territorial?
  • What are the needs of this particular fish? Will it need special plants or other life forms to keep it happy?
  • What size tank will this fish need?
  • Is this fish easy, medium or difficult to keep?
  • Is this a hardy species, an average species, or one prone to picking up disease?
  • What water conditions does this fish need? Are the pH and specific gravity ranges compatible with your other fish?
  • What optimum water temperature does this fish need? Is this compatible with your other fish?
  • Does this fish require low, medium or high levels of light? Is this compatible with your other fish?
  • What does this fish eat? Is it an herbivore, omnivore or carnivore? Is its food easily available to you?
  • Is this fish a nocturnal feeder?

Juvenile or Adult?

It's likely that the majority of fish you add to your tank will be juveniles, because they're less expensive and the most readily available. This gives you two additional things to think about: the size of your species and the color. Consider the following before you buy:

  • How large will your new purchases become?
  • Will your tank be large enough?
  • Juvenile fish will adapt better to your tank. Some smaller juvenile fish are less likely to be attacked by the adults.
  • Juvenile fish often display radically different colors than the adult version of the same species. Ask before you buy!
  • Be wary when buying adult fish. Adults may be proven breeders, but they may be at the end of their breeding capability.