Hot tub water with very low total hardness (very soft water) can cause etching to occur on the tub surfaces which will become abrasive and uncomfortable for bathers. Rough surfaces also increase the likelihood of problems with algae growth and make cleaning more difficult. Hardness levels should be maintained between 250 – 500mg/l (ppm). RotoSpa Hardness increaser granules are used to do exactly what they say on the tin. Increase the hardness in your water to help stabilise soft water.