yeah- getting them to puff up and display won't shorten their lifespan unless they're so aggressive about it that it freaks them out and makes them stressed. It's actually a good way to make sure their fins are intact and to check on their health. And they do it on their own all the time anyway- whenever they see something that might be another fish or something to fight or eat. So yeah- a red pen top- or a bright colored feather- or anything roughly fish sized and brightly colored will generally set them off.