Because of the low bio load of the LR
in the smaller tank it will pretty much be devoid of organics. In which case I'd consider it no different than buying 10lbs of cured LR
from your fish store. I'd put it in your 75 and be done with it. Maybe check ammonia a few times over the first few days.
is growing all over the LR
in the smaller tank it might already be just inert rock with little biological benefit. In that case a rinse off in fresh water should clean it of the cyano