your research first. because if you buy your aquarium, you might have a very vague idea about what you want to put into it. but once you buy it, you are very limited to the options of what you can do with it. except, of course, if you buy a huge home aquarium, then you can do pretty much everything you want with it. but if you are just starting out, most likely you will want to “test the waters” and see if this hobby is for you, right? so, most likely, you will go for a smaller aquarium. but once you do that, your options are limited.
if you know 