While I was on vacation I went to a hotel that had an aquatic science center. They had two saltwater tanks that really bugged me. The first was a 55 gallon tank with a longnose hawk fish and a young chevron tang. The other tank was about 70 gallons with a dozen damsel fish and a wrasse. Both tanks had nasty brown algae. Then I saw a tank with a painted turtle. The fish being used as feeders were neon tetras.
Rule number one: Always research a fish before you buy it.
Rule number two: Always cycle your tank.