Will Not Affect Ph Or Alkalinity, Allowing You To Adjust Each Of These Any Way You Wish.Super naturals by caribsea our pre-washed super naturals are designed with your success, and ease of use in mind. packaged in sizes that make sense, most types are available in both 20 and 50 pound bags. we have carefully selected different sizes and colors for you, to help you create a natural underwater environment. these inert products will not affect your ph or alkalinity, allowing you to adjust both of these important water quality parameters any way you wish. while these can be used for marine aquariums, keep in mind you will have to use standard buffers, our aragamilk, or aragamight to keep your alkalinity and ph up where they need to be. amazon - an inert, natural substrate with rounded grains. recommended for freshwater aquariums with external filtration, or undergravel filtration. suitable for planted aquariums. grain size 4.0 - 6.0 mm.20 lbs