In the absence of other filtration (cannister, hob
, wet/dry) a tank requires 1 - 2 pounds of porous rock per gallon of tank capacity to house the bacteria that turn ammonia into nitrite and nitrite into nitrate.
If you never add fish you can use much less rock. However, the fact that the rock was fully cured and the EMPTY (of livestock) tank appears to have cycled does NOT mean that you have a sufficient population of bacteria to handle the bioload once you start adding fish.
As the bioload increases there will not be any additional surface area for the bacteria to colonize. That will cause a build up of ammonia in your tank and lead to MFD (mysterious fish death/disappearance).
So, 50 pounds of LR
in a 120g tank just won't cut it unless you have some other method of filtration working for you.
You could also do daily 10% water changes and keep the tank lightly stocked (2 -3 medium sized fish).