The most commonly used mediums are Rockwool or Hydroton grow rocks.  Rockwool is usually sold in squares with or without holes or slabs. It is also available in many sizes for you growing and cloning needs. Rockwool is made from, would you beleive rock. It is very porous and holds massive amounts of oxygen. Oxygen is one of the  factors that contributes to the accelerated growth attributed to hydroponics. Rockwool should be soaked in water @ a Ph of 5.0 overnight before planting a seed or clone. Do not let rockwool dry out and always use good quality hydroponic fertilizer. Transplanting is easy with rockwool, simply put a bigger block or slab underneath or place into grow rocks.When starting from seed in rockwool use diluted fertilizer designed for grow, and not bloom, use bloom when cloning. Also mist with warm water as necessary and put on a humidity dome or clear plastic bag-wrap over the top.   I recommend useing Olivia's Cloning Gel. Discard used rockwool, it cannot be reused. Grow Rocks are round clay fired balls about 1/4-1/2 inch wide. The rocks also hold much oxygen and are popular because they also provide good support for plants. To use grow rocks soak in clean water so all dust and chips from shipment settle to the bottom. Repeat until completely clean. Grow rocks can be reused by boiling them in a solution of 10% bleach to 90 % water and laying in sun, or in an oven on warm to dry. If you thnk it is too big of a pain to get all the previously grown roots out lf the rock then discard them.