kenny,im not certain but may have your answer.im sure britta filters filter out metals etc from the water,they possibly[im not 100% on this but ur tale supports the theory] filter out minerals,including'micro'and 'macro'nutrients[nutrients the plant uses but in lesser quantitys]and as you say its taken a rams horn shape;this is often the sign of nutrient deficiency...epsom salts is cheap and if this is ur prob,a tea spoon full in 5litres of nutes/water in the first week will serve well.