Fish mate in pairs or groups where a female lays eggs that are in turn inseminated by the males. Knowing if your fish are mating requires some basic observation and deduction skills. Your fish will only mate successfully in an aquarium if you have a male and female. In most situations, you must have the same breed for successful mating although some crossbreeding is possible.
Your fish will show signs of mating before the actual mating happens. Look for fish that pair off or groups of males showing interest in a female. In the case of the silver dollar fish, the males will aggressively chase the females around the tank. In other species like the parrot and oscar fish, males will against the females and wiggle or vibrate their bodies. In some cases they will lock lips as well. This flirting process is the beginning of mating and the nest will soon follow.
Building a Nest
The female will begin building a nest when she is prepared to mate. She will fan out an indentation in the gravel, creating a safe space to deposit the eggs. Some species will drop eggs in sand and others will drop the eggs without a nest. The nest-building process is an obvious sign of mating. When the female has an adequate nest, she will select a mate if she hasn't already.
The actual spawning process begins when the female drops her eggs. The female will position herself over the nest and the male will rub up against her. Eventually, the female will deposit her eggs and the male will fertilize the eggs. The number of eggs varies depending on the species but you will typically see the eggs as she deposits them in the nest. The exact time between laying and hatching also depends on species. The silver dollar fish will hatch within a week of being laid while some species take more than a week.
After the eggs are deposited, the female will defend the nest. Parrot fish will have both the males and females defending the nest but many species rely solely on the female. Other species in the tank will target and attempt to eat the eggs. You may encounter a few fights between the fish as they go for the eggs.