Research How-To: Mineralized Soil Substrate, by Aaron Talbot
Adding clay soil to the washed substrate will help bind the particulates.
The submersed soil should clear within 6 hours. If not, wash additional times. Washing the soil multiple times will not eliminate the organic material.
This could take several weeks.
Some top soils will never clear, especially those found in the yard. It is better to discover this outside of the tank than inside.