Before you set up an aquarium, there are a few things that you should consider, these are:
Type of fish you wish to keep, saltwater or freshwater, tropical or coldwater
Size of aquarium you can have e.g. space permitting, budget and aesthetics
Time you can commit to your new pets
It is always recommended that you research the animals that you wish to keep. Understand their needs and requirements, understand the basics of water chemistry and how to maintain the environment in which your animals live.
Unlike most animals, fish have to do everything in the water that they live, so breathing, eating, pooing and weeing in the water. They can't go to the toilet and flush it away, they have to swim in it until we remove it for them, if they don't eat all their food, they can't throw it in the bin, it stays there and will rot away until we remove it. Therefore, they are reliant upon us to keep their habitat clean for them.
Therefore it is vitally important that we select the right environment for the fishes and the right fish for the environment that we have given them. The tank must be large enough for the fish to swim around, but also it must be large enough to handle the waste that they produce between cleaning. We must also provide filtration to filter out as much waste as we possibly can, this helps to keep the environment clean.
We must also provide them with the right temperature where required, and lighting, these vary greatly depending on the type of fish you wish to keep.
Marine fishkeeping is widely regarded as the pinnacle of fishkeeping, and as such does require more in terms of maintenance and understanding of water chemistry and the environment in which they are kept, the cost implications of marine fishkeeping is a lot more expensive than tropical or coldwater, but that said is a lot more rewarding and pleasing to they eye.