The correct way to dirt a tank is the cap it with gravel, sand, or anything else you want to really. This will hold the soil down, and stop it from floating into the water column. Then, fill the tank GENTLY. You don't want the force of the water to be cutting into the gravel/soil. Use a dish or something to catch the water force. You should have very minimal, if any, soil in the water. My tanks are 100% clear the very first day.