If you are getting scale in your pots means the water is hard (alkaline). Alkaline water makes the soil alkaline. Alkaline soil is not liked by bacteria which help in nutrition. Alkaline soil is loved by slugs. Why not just make it into a practise to add vinegar to the watering can and drop PH to 6. Helps in making soil acidic(attacks shells of slugs). Also think of adding elemental Sulphur to soil which in turn over time makes the soil acidic plus any of the other sulphates eg. magnesium sulphate makes the soil acidic and provides magnesium at the same time. You can also use dilute battery acid (sulphuric acid) if you can get it. Nitric acid dilute provides nitrates.
Just my two cents worth.
Regards,
Masud.
