Just Random?

With the Ebb and Flo (or whatever its called) method do you have to use specific rocks, pellets whatever or could you go outside and get some (for example) crushed limestone and bleach them and just use that?

So I take it that all you Hydro Buffs must not know if I could use bleached gravel from outside or fish tank rocks or something along those lines as a medium. Thats cool.