70lbs for $15? That's a great deal! When you say dead LR
, do you mean they just had it sitting out of water for a long time or something else? Once you have scrubbed the rock of any visible junk and the levels in your curing tub reach 0 ammonia and 0 nitrites then it will be safe to add to the DT. The bacteria is not what causes the cycling of the rock. What causes the spike in amm. and nit. is the decay of organic matter (dead sea critters). The nitrifying bacteria then grow on the rock to convert the amm to nitrites, then nitrites to nitrates.
The point your at now, it would be safe to start adding the rock in 5lb increments. Add 5lbs, wait a few days to make sure your levels are fine, then add 5 more etc.If you add it all at once, it could cause a mini cycle