W/ LR, not necessarily. It may even be ready to handle live stock now.
Check the water for pH, NH3, No2 today and another one couple of days later. If both reading revelas proper readings (0 on NH3/NO2), should add (1) small fish to keep the LR going.. after adding fish, Test every other days for NH3/NO2. With (1) one small fish in FOWLR, should not encounter any problems.
I used to add fish on the same day I add my own LR which were cured in my curing vats in order to keep bioactivities going in/on LR
Since only 12 G, should think hard as to what you want to keep.
Good Luck!