There are two main strategies that fish use: egglaying and livebearing. 

Livebearing fish do what the name suggests. The female gives birth to fully formed, free-swimming young. The female fish is internally fertilized by the male fish, and carries the fry for about a month before delivering them. Upon delivery, the babies swim off, hide, and begin searching for food.

If you're looking for a really unusual addition to your tropical freshwater aquarium, hatchet fish might not immediately catch your attention. Although they have an unusual body shape they're rather plain in colour and they're often shy. But hatchet fish have a secret - they're the only species of fish in the world that can actually fly.

Unlike other so-called flying fish which are merely good at jumping, hatchet fish actually flap their specially shaped pectoral fins to help them move further through the air. 

In the wild, this helps them to catch flying insects, and some aquarists breed fruit flies so they can feed them this way. Of course, you'll need a secure aquarium hood and a good bit of space above the water surface to keep them safe, but they're amazing to watch.