Yes, Alison I use the kalkwassr, and I do the settling down of debris on the bottom. From what I understand, these are the impurities and excess pickling lime settling as one litre of distilled water only holds so much pickling lime, with the remainder settling down. For salt water aquariums, it is even suggested to not filter out the debris as it continues to draw out impurities from the water.

My concern is around the 1% impurities(arsenic being part of that 1%) and the possible toxic effect of the accumulation of such impurties in the body cells and tissues over time. Food grade pickling lime may be a safer option as there is much more effort taken to filter out such impurities.

I am going to try and switch to food grade pickling lime if I can get my hands on it. My gut feeling/intuition, for what it's worth is telling me that it may be a safer option.

This article nicely sums up what I was talking about when it comes to kallwassr.

http://reefkeeping.com/issues/2005-01/rhf/index.php