Hot tub water with very low total hardness (very soft water) can cause etching to occur on the tub surfaces. This will become abrasive and uncomfortable for bathers. Users with sensitive skin can suffer due to this issue. It can cause itching and rashes to bathers.
Rough surfaces also increase the likelihood of problems. In particular with algae growth and make cleaning more difficult.
Hardness levels should be maintained between 100 – 200mg/l (ppm). Never add more than one chemical at once. And never mix chemicals.