Hard water stains are not permanent on my trees, because the rain washes it out. If we get any rain.
If it gets too much, I just drop my tree in the pond for an hour or so, instead of watering it like usual. After a couple times of watering like this, the buildup is usually gone.
If you have the possibility to get your hands on a rain water barrel, you don't have to worry about pH levels, filters or salt buildup ever again.
Filter attachments sound sketchy. I know about polymer filters that people use for their laundry and coffee machines, but those devices are pretty big and pretty expensive. I don't know about hose attachments but since those in-house filters are roughly 50cm in length and at least 5cm in diameter I suspect a hose filter to be of a substantial size as well. A friend of mine bought "filters" from some kind of pyramid scheme, those were filters one could magnetically wrap around a hose and those would stop the salts somehow, both chemically as well as technically, that's pretty much impossible. If your filter works in a similar sense, I would ask my money back.