Is there something radically different about septic tanks in France? In every house I have lived there has been a septic tank( UK, Canada, Oz etc) and I have carried on as usual with the Bleach and whatever cleaning products I fancy with absolutely no problems. Recently, the new type thicker loo roll did cause a minor problem so stopped using it but I don't think any of the bleach products I use have changed their chemical make-up! I really would like to know as I am moving to France before the end of the year. Thanks