I would guess they are thrift flies also known as gnats and a billion other names if they are small and black they came with your soil, they eat plant roots and wont harm your frogs infact they are a good food source for younger ones , to avoid them the only way that's safe for a frog would be to cook your soil in the oven at 350 for about 5-10 min before you put it in their tank, do you have a picture of the bugs would be easier to identify that way, but if they are gnats they will go away on their own when the food source is gone often mold or plant roots or decaying plant mater is what various species eat. as for tank i would go the half and half approach FBT's like to spend a lot of time at the water edge, they will go explore the water and hunt up on land but when they rest they like to hide in some plants floating in water, if you could give them a room with a pond in the center you old find 90% of them less than 2 feet from the water