Use styrofoam packing peanuts instead of pebbles or rocks for drainage. If the pebbles have limestone, they'll change the ph of the soil as the lime leeches into it whenever you water. You can save the peanuts from packages or buy bags of them where you buy office supplies, home improvement stuff, or UPS, FedEx, or USPS. They're reusable too. Just knock them off the bottom of the soil when you go to repot. If a plant grows long roots, it can grow right into the peanuts without any problems.