If conditions are right, life grows exponentially. 2 4 8 16 32 64 128
And that's just the first week, if there is proper food and surroundings. How much life was in the sand from your old tank? Were there worms and grammerus, copepods and rotifers, bacteria and protozoa? Did you rinse the old sand or leave it cruddy? What larger organisms are living in the new tank? How about the rock?