12x12x18 is good for a very short period of time. They should be ok in there as long as a move to a 18x18x24 or similar tank is done sooner rather than later.

I use 12x12x12 and 12x12x18 to grow out froglets until they are around 3 months old, but would not house sub adults or adults in them, just to small for them to be able to get away from each other if they need too.