Common hydroponic aggregates. Only materials that have a higher air to water retention ratio are included.
Growstones are a substrate for growing plants that can be used for soilless purposes or as a soil conditioner. This substrate is made from recycled glass. It has both more air and water retention space than perlite and peat. Another property of this medium is that it holds more water than parboiled rice hulls.  Growstones appear to be a comparable alternative to expanded clay aggregate.
Parboiled rice hulls
Rice hulls that are parboiled (PBH) are a substrate or medium for gardening, including certain hydrocultures. This medium decays over time. Rice hulls allow drainage, and retain less water than growstones. A study showed that rice hulls don't affect the effects of plant growth regulators.
Wood fiber reduces the effects of growth suppressant hormones, used for uniform growth.
Coco fiber is a common hydroculture aggregate made from coconut hulls.
Gravel is good to be layered on the bottom. It allows better drainage near from the plastic net-pot. Placing seeds too close to plastic allows them to rot easier.
Aquascaping specific substrates
- (2011). "Growstones ideal alternative to perlite, parboiled rice hulls". American Society for Horticultural Science http://esciencenews.com/articles/2011/12/14/growstones.ideal.alternative.perlite.parboiled.rice.hulls
- Evans, Michael (February 2011). "Physical Properties of and Plant Growth in Peat-based Root Substrates Containing Glass-based Aggregate, Perlite, and Parboiled Fresh Rice Hulls". HortTechnology 21 (1): 30–34. http://horttech.ashspublications.org/content/21/1/30.abstract?sid=92f8e649-8297-40dd-b158-c0e98baa6dfa.
- Wallheimer, Brian (October 25, 2010). "Rice hulls a sustainable drainage option for greenhouse growers". Purdue University. http://www.purdue.edu/newsroom/research/2010/101025LopezHulls.html. Retrieved August 30, 2012.