you'll need at least 5-10 gallons to house your fish ( if considering buying a goldfish, or Betta fish, 4- 20 gallons, depending on the max. size, some get dang big, like 2', so ask the nice employees at petco!:) and they dont need heaters.). i have a ten gallon aquarium and it's just great, and only around $60 So go on petsmart.com or petco.com and find which size, shape and color is right for you. Once your at the store and have decided which tank you get, you need a heater and filter, and possibly a light, but most aquariums come with it, and don't forget the most important thing. Fish! I can't stress enough how important it is to do research on the fish you want to get.You do not want to sit at the pet store, to find out that " That one gets two feet long, or "That one is a saltwater fish" or "that's a cold water fish" and blah, blah blah. so research! (refer to step 2 for advice on fish)
So...you have to actually trim their teeth by knocking the fish out and using nail clippers if they haven't been eating enough hard things.
I thought about keeping some until I found that out...
http://www.thepufferforum.com/forum/library/hospital/dentistry/
1 = 30g
2 = 40g
3 = 50g, and so on.