3 1/2 monthes? Is it not cycled? 50lbs is not enough to handle a normal load IMO
. but it will cycle and you will get nitrifying bacteria but IMO
it will not be enough to handle the normal load of a 75 gallon tank. So yes IMO
you could cycle the tank and add a fish or two but you will need to add more LR
to get the full benefit of the size of your tank. I would keep a very close eye on the ammonia levels as this will be your indicator! Hope this helps, and hope someone will correct me if im mistaken.