Hi there
I will try to answer as best I can, – (unfortunately as you may know, this forum is not very active, )
It sounds a bit strange what you describe.
First of all, its not age for sure. Paros can get older than a couple of years for sure.
It can be different things, – water, the filter, contamination as I think you wrote, obviously something has become different since you moved ?
It seems to me to be something related to the water, – since the fish are itching – rubbing their gills, – some kind of poisioning. But from where it comes is difficult to say.
The best thing I think you can do, is what you are writing, create a completely new environment. However, I would really not think that you should treat with any kind of treatment. In fact – these can make thing a lot worse. Medication is often extremely unpredictable, in particular in soft and acidic water, – it may work much harder, – and paros and no fish that takes medication well.
So even salt, – personally I would not treat at all.
Create the best optimal water for paros, – soft, acidid, and let them be in that