Just to be clear - I really don't think used fixer, in home darkroom quantities, would pose any risk but I probably personally wouldn't discard it into a septic tank anyway out of an abundance of caution. Many toners certainly shouldn't be poured down any drain, septic or sewer system. Developers - well, it depends, but if you aren't comfortable looking up the ingredients and deciding certainly one can err on the side of caution. I was just raising an eyebrow at the ludicrousness of including stop bath in the list.