ok i filled my 75 gallon tank and it looked fine but one day later this stuff was on the bottom it was white i do not know how to describe the texture of it but it looked sort of "powdery" i guess you could say so i got my gravel vacuum and got the majority of it out has this ever happened to you this is the first time it has happened to me and what is it and is it bad for my fish pleas help i am very scared that it could harm my fish