Are aquarium starter kits only for freshwater, or can they work for saltwater too?
Some starter kits are built specifically for freshwater while others are made for ocean fish. Check the aquarium type in the kit details, and make sure the filtration and lighting match the environment you plan to keep. Whatever you choose, plan to watch water quality closely so the system stays stable from day one.
Which parts of a starter kit are most likely to need upgrading as your tank matures?
Filters and lighting are the parts most often upgraded if the included hardware is not keeping water clear or bright enough for your setup. Many kits focus on the core essentials, so you might also add a heater for tropical setups or extra filter media as you add more aquatic life to your tank. Regular water testing will tell you when performance needs a boost.