I use coal slag in my planted tanks. Its essentially black sand like you see in a LFS store. Its actually used as an abrasive in sandblasting. Its PH neutral, looks nice and works very well as a substrate in planted tanks. It's like eco complete without the nutrients.
http://groups.yahoo.com/group/caga
If you go to the above yahoo group, you can see the planted tank on the initial page uses it as substrate. If you look in the photos section, there is a section titled "Chris Michael Tanks) to see what it looks like. I'll get some pics if I have a chance.
I've been using it for about 6 months and its worked out fine. Its about $13 for 100 lbs. Its mentioned in the krib site as well.
It comes in varying grades, I use the medium grade (1240).
I posted previously...the Dojos love it. I was careful for the first couple months to check daily to see if there was any damage to fins or barbs, but no problems showed up and it really looks nice.
The plants seem to like it too
Kevin