get some CHEEP top soil from your home building store of choice, one bag should do, about two bucks
toss it in a big bucket and add water to flaot out the mulch and wood, spread on a tarp and dry in the sun. wet and dry seveal times. this mineralizes the soil. toss back in said bucket and strain out the gravel, I used an old fishing net
what you are left with is a very fine sandy dirt, to this add a fist full of clay, I used potters clay, $1 a pound
sprinkle a spoon full of potash on the bottom of the tank and a quarter of a ground up cuttle bone(the kind you hang in the birds cage...it is dolamite(?)
then add your top soil and cap with swimming pool filter sand purchased for 6 bucks for 50 lbs.
easy and cost effective! here is a thread that goes into the details....