I've researched this and tested every inline shower filter I could find including;
KDF filter - doesn't filter calcium
No salt water softeners - makes the phf even more alkeline (zurich water is about ph8 this pushes it up to ph9)
Hyundai ion exchange - works well but for max 1 month (says on pack good for 6-9 months) and cost 30CHF each
Portable salt water softener - refills with 1 cup table salt in 1 litre water, leave for min 30 mins and run shower for 2 mins before use. Lasts about 3 days for 2 people before refilling with salt - best option I've found, comes with kdf filter as well and I added a Hyundai ion exchange as a last resort filter.
I've spent about 18 months trying all of these and the last solution is working the best. Hope this helps someone, it's made a big difference for me as the water felt like it was dissolving me without the filters.
Links to solution I'm using: http://www.watersticks.com http://www.amazon.co.uk/HYUNDAI-wate.../dp/B00CJA3E16