protein skimmers clean the water. it's never a bad thing to have one. if you are judging your need for a skimmer by how your fish seem to be doing, you are making a mistake. if you need a skimmer (this will be determined by your current bio load, along with your feeding and nutrient export methods and frequency) you won't see any signs from the fish until it's too late. when the fish start failing, the water will be toxic and then you and your fish will be in a terrible position.
what size tank?
how many, what kind, and what size are the fish?
how much and how often do you feed?
how much and how often are your water changes?
what if any, other methods of nutrient export do you utilize?