As an avid fishkeeper I am often plagued by the question, how big is too big? Or, more accurately, I have an addiction of wanting (and getting) bigger and bigger aquariums. I feel that many people are given the wrong advice when first starting off fishkeeping. The general rule of thumb that I'd say is the safest is: start with as big a tank as you can afford. There are several things to keep in mind, of course, when selecting the proper set up for your fish. First, what kind of fish are you wanting to keep? Marine, freshwater or brackish fish? Second, how much space can you alot for your aquarium.
For marine tanks, most definitely the bigger the tank (55 gallons plus) the more stable your fish community will be and this is important because this means less disease, less territorial fighting and less maintenance you will have to do. There are also less salinity fluctuations which are vital to marine fish and it's hard to control in a smaller tank like 10 gallons.
For freshwater tanks the beginning fishkeeper can get away with a small 5 or 10 gallon set up and be successful - provided one has all the essentials like a filter, proper lighting, a heater for tropical fish and maybe a bubbler for added oxygen and no over crowding of fish! There are kits sold in petstores that come with only a filtration unit. Avoid these "cute", compact tanks because they do not include heaters. For goldfish, a heater is not necessary but it's a nice commodity for them, they can live in warmer water than room temperature. For tropical and marine fish a heater is the means of life or death. If the water gets too cold the fish will die or get diseases. As stated earlier, the larger the tank like a 30 gallon freshwater, will have more stable water chemistry and less fish disease. Regardless of the size of your aquarium you can enjoy your fish.
Even a small 5 gallon aquarium can be a wonderful learning experiance as its own ecosystem. Featured below is an exampe of one of my newest additions, a 5 gallon hex aquarium. It is a small community tank with a "faux" marine theme that is freshwater fish only.
So, if you have a small budget or a big one, fish keeping can be very rewarding at any size.