What Size Fish Tank Should I Buy?
Unfortunately, many people buy their fish tanks before they buy their fish. Whilst this seems to make total sense (where are you going to put the fish if you don't have a tank for them?) It can be misguided as people buy tanks without having thought about what fish they are going to put in them and end up with fish that are incompatible with the tank or with each other.
Ideally you should start by thinking about what kind of fish you want to keep, and then going from there. This article is a rough guide to tank sizes and what you can put in them.
5 - 10 Gallons
In my opinion, these tanks are really only good for very small fish (like a few neons or a couple of female fighters, or as single tanks for male fighters.) They're good for children who just want to keep one or two small fish as a learning experience, but if you're an adult seriously wanting to keep a few nice fish in good conditions, you need to go larger.
20 Gallons Or Less
You should be looking at keeping smaller fish in a 20 gallon. Neons, guppies, mollies, that sort of thing. 20 gallons really is the lower end for any fish keeper. This may be frustrating to hear, but trust me, if you buy a 20 gallon and develop a passion for fish keeping, you will be well on your way to multiple tank syndrome before you know where you are.
55 - 90 Gallons
These are the sizes I would recommend for the beginner who wants to keep a nice community tank.
The 55 gallon is a good medium size and you can happily keep medium sized chichlids and gouramis in here. You can also keep one of the larger chiclids. And I do mean one of them. One Jack Dempsey or similar fish will do okay in a 55 gallon, but you won't be able to put much else in there. For this reason, I would recommend that you keep medium sized fish that grow no larger than three inches in this size tank. You will be able to fit in several pretty chiclids or gouramis (probably not both) as well as some shoaling fish.
If your tank is tall enough, you can keep a few discus or angel fish (again, not both, they are not very compatible together, though some experienced keepers can get away with it. )
If you want to keep a pair of the larger cichlids, you will definitely need a 90 gallon. If you want to keep lots of medium sized fish, a 90 gallon is ideal and will make a beautiful show piece.
100 Gallons Plus
At 100 gallons plus, the world is really your oyster. You can keep monster fish, such as Arowana, Oscars, Dempseys, Birchirs and many more. Be aware that you will still be limited by compatibility issues, so make sure to research your fish before you buy them, most of the larger non shoaling fish are fairly aggressive (with the exception of plecos and catfish.)
Research every fish before you buy it and ideally, plan your tank before you buy a single fish. There are a lot of resources online which will help you learn about fish and their various traits, and having planned a tank in advance will make you a much more successful and less stressed keeper.
Keep in mind, you cannot plan a tank, buy it, and stock it full all in one go. It needs to cycle, otherwise you risk losing all your fish.