You can actually get custom aquariums and the fish to populate it all on the same day however this is not a good idea. You should spend some time to set up the aquarium perfectly. You will need to make quite a few decisions not least of which is going to be whether you are going to keep tropical fish or that of the freshwater variety.
You need to ask yourself a number of questions which include how many fish would you love to keep? Remember that fish could grow in size and they will need more space as they get older. Also some fish are sociable by nature so like to swim in shoals and not live separately and therefore this should be taken into consideration when getting a fish tank. Custom aquariums are available in all shapes and sizes so you will need to make a choice of how big you like yours to be.
What type of food will the fish have? How simple is it to get this food? You would like the theory of owning fish that eat live food but can you live with the reality? Will the fish you have chosen share a tank with other fish or will they eat or pester each other? Do you want to breed fish? In that case have you chosen those species that are known to breed in captivity?
These are just some of the questions you have to ask yourself. If you have decided on the type of fish you require you will then need to work on the decor of the fish tank. You shouldn't put just anything into a fish tank. It needs to be suitable for use with custom aquariums or you could unknowingly put your new pets in danger of infection, disease and death. If you are not yet knowledgeable, take advice from someone who is.