Im not entirely sure about what you specifically have; but a permaseal is/should be removable. You'll likely need to use an allen wrench to do so. Whether or not it has to be removed prior to filling; I'm not entirely sure and would leave that decision up to whoever it is that fills your tank for you!
On the other hand? I have not used one of these as they seem to be a bit more nuisance to solve a problem that I don't find. Is this something that you have found helpful or only something suggested that you have not tried and may not need? I will not try to change your methods if it is something you like but then if it is new, I might suggest that the simple nylon washer works really well and lasts a long time.
Much depends on how you seal it and using a good wrench to get the torgue needed to compress it well.
Question solved if it is not used?