It is a red baked clay used for infields and golf courses. It replaced soilmaster select which is no longer made. And the charcoal color is no longer made. It looks like dark red fine gravel.

I costs $8 for a 50 pound bag. I got a local supplier by emailing the factory.

see:

Pro's Choice Products

.02

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fw leiden since 1979, fo salt since 1979, mixed reef 55g 2002-2009. Strong emphasis on the tank taking care of itself. Balanced with plant life, no water changes, tap water, no filters in FW. Only dosing calcium, alk, mag in marine reef tanks. http://www.aquariumforum.com/f15/my-...ods-26410.html
recent tanks (till 2009) 7 years- 10g FW leiden 7 yrs, 55g mixed reef 7, 2 yrs, 20g FW leiden, 10 g fw leiden , 29g mixed reef, current tank 55g leiden