Hello and welcome to FF! Providing a proper substrate for ADF can be challenging due to it's eating habits. Those uneaten pellets decompose in the water fouling it and if your frog finds and eats them it could get sick. Myself would check your nitrates levels if tank is cycled and if newly set-up would also check ammonia and nitrites in case increased water changes are required.

Best option with ADF is a bare bottom; in your case, just need to carefully remove the pebbles with a net. Then you feed the food and frog finds it with no issues. Also, it's easier to keep tank clean.

Second option is to use a fine layer of aragonite sand (no more than 1 in. deep) as substrate. If frog accidentally eats it; it will go through digestive system with no impaction issues. Problem is that powerful filters can pull fine sand it into media pads and also disturb the bottom layer with outflow. Good luck !