Native to the warm waters of the Great Barrier Reef and similar oceanic areas, the clown fish is a colorful and attractive saltwater fish. By carefully observing the behavior of clown fish in your aquarium you will be able to tell if they are breeding.
Observe the behavior of the tank mates. Ordinarily, the female of the mated pair will assert dominance and attack the male, nipping at his fins. This is a sign of sexual maturity. If the male submits to the female, both become ready to breed, although no acts of breeding may be witnessed.
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Pay attention to the appearance of the female clown fish if you suspect the fish have mated. One of the first signs of pregnancy includes thickening and swelling of the fish's midsection as she becomes full with eggs.
Observe the behavior of both the male and the female fish. If the fish constantly appear to be cleaning off rocks and digging burrows into the gravel near secluded parts of the tank, this indicates pregnancy.
Continue to observe the appearance of both of the fish, daily, to determine whether or not the female could be pregnant. When the clown fish becomes ready to spawn, both the male and female feature tube-like protrusions from their underbellies. These tubes allow for both the spawning and the sperm-fertilization of the roe.
Pay extra attention to the areas in which the clown fish have cleaned and cleared out debris in the tank. If a female is carrying eggs, both fish swim rapidly for hours across the surfaces of rocks and tank decor objects just before the eggs are laid.