I'm currently using about 30 lbs of lava rock, (I have 3 large chunks in there that make up most of that weight, so, the tank doesn't look like it's saltwater BBQ grill

), in a seperate tank that just has some shrimp in it, and that's it, (no snails, etc). I have noticed a little bit of a brown algae / slime build up on the glass, but, I am only running a sponge filer on it, to keep the water in motion, (and haven't done any water changes, or cleaned the glass). This has been running for 3 months now, and that's the only complaint that I can give you on using lava rock. Granted, I haven't tested it for anything else, but, the shrimp are happy, and reproducing, so, can't complain.
Let us know if you decide to use it and how you make out with it, (I rinsed the lava rock in a 5 gallon bucket for about an hour, to make sure that all of the smaller lava dust / pieces, were removed, and the rock was as clean as I could get it). HTH.
Robert