I was quizzing the employee at PetSmart the other day, and I was informed that an oscar needs a 20 gallon tank when fully grown, and a common pleco needs a 30 gallon because they don't make 24 inch aquariums, and plecos can grow to 2 feet (the lovely one inch per gallon rule). Oh, and all plecos grow like this, so my BN is going to grow to 2 feet, too. Oh, and I could have 2 bala sharks in a 30 gallon, and goldfish need 5 gallons per fish.
The only questions he answered right was 'Do fish grow to the size of their tank?' (Not really, its very unhealthy to stunt them.) and 'Can you mix goldfish with tropical fish?' (He explained that goldfish were really messy and other fish couldn't handle thier messiness, so no, they should be kept seperate. Right answer, but not the explanation I was looking for...)