I was just reading a limnology book at the lib yesterday and for a gas in solution (the example used O2) to diffuse 10m in a lake that was otherwise undisturbed takes about 600 YEARS. So forget this mm per day stuff. Their point was that diffusion is VERY SLOW. OTOH, if you have even Malaysian snails in your substrate, you will have more than just diffusion going for you. Dave Gomberg, Experimenta San Francisco CA USA gomberg at wcf_com