You can buy Picking lime (food grade Calcium hydroxide) at HEB (Mrs Wade Picking lime is the brand that I use) It is really in expensive. Only seasonal about this time of the year. People use this in food canning. Because the "self purification" process of kalk (precipitation of impurities) and I feel that If I can eat it, the fish can too. It cost about 1-2 dollars for 1/2 a gallon powder of this stuff. i have used it for 10+ years without problem years