Aerator
Aerators are a "must have" for anyone who hopes to run a successful aquarium. No fish can thrive without an aerator. Some pet stores advertise that fish like Bettas and Goldfish can, but that simply isn't true. Some aerators may come in the form of "air stones", submerged into the gravel, while others are shaped like aquatic structure (see photo above). Air stones take up less space and work well in small and large aquariums. Stucture shaped aerators take up more space and look better in tanks 10 gallons and larger.
Heater
Tropical fish will need a heater. Your heater should be the correct size for your aquarium. It will likely say on the package the heater came in the size aquarium it can heat. There are two types of heaters. The most expensive heaters offer a control switch, so you can ajust the temperature in your aquarium. This may come in handy when you are trying to breed fish, as some fish only breed when the water is a certain temperature. The less expensive heaters have no controls (see photo above) and heat the aquarium to a certain temperature. When it reaches that temperature, it automatically shuts off.
Filteration System
Filteration is essential for any aquarium. They make sure the water quality reasonably good. If your tank does not have a filter then you will have to do water changes much more frequently. It also keeps the nitrogen cycle going, which is essential for your fish to surive.