I wouldn't consider pacing as a sign, as they don't pace... they just swim around. Some just have preferred routes.
Luckily, they don't get "bored" in the terms that we know it as- as they are fish and their minds don't work that way. It's if the tank is bare then they will hang around more then swim, if it's got a good amount of decorations, or if they have a neighbor, they will swim around often. Unsure about their memory span.. I remember a goldfish's memory span is only 3 minutes.
Having things for them to do is always good, but don't worry or stress out thinking they will be "bored" as they literally can't comprehend and establish boredom in our sense. There is just either places to explore, or not and they react to that, the surroundings, rather then what there is for them "to do".