You can add an inline one it will only make it better, but if you have one you may as well set it up in you tank just remember parasites, viruses, algae and bacteria (good and bad) are the types of things that are "killed" after passing through the ultraviolet. UV
kills even the good .You could also do the full clean and then do a water change after by just moving the pipe to the bucket or clipping on a long hose to push it out a window or into the toilet,sink.When you are done cleaning the gravel do the water change to remove even more as they may be floating around in the water you moved around if they did pass the filter but it quite sound. Do the gravel clean a few times to get it started and after a while you will have removed most if not all but they are small. I only have fresh water tanks so its a bit easier for me.