I use green zucchini for raising isopods ( they love it ) in a separate container w/ loose chunky soil.
Once they are in the tank w/ plants, roots, wood and frog poop etc -they will nothing else.
Springtails, on the other hand , are easier, reproduce more quickly ( I think), and require only live bakers yeast and loose soil.
I use ABG in both the springtail and isopod cultures. When I want to remove them... I just scoop some out/ and replace the soil as needed.
You might seriously consider a quarantine period for your new frog.
In a small tank ( ie 10 gallons) w/ plain paper towel substrate( dampened with de-chlorinated water)
get to know him-
keep that close watch on him.
This is and ideal opportunity observe his habits, monitor his food and poop. ( yes poop)
You will be able to address any issues immediately !
This small tank will never go to waste when you keep frogs.
One should always have a hospital tank and some basic first aid supplies on hand.
Is the 'pet store' a herp store specializing in lots of amphibians and reptiles or is it a place that gets one or two frogs now and then?
You want to be alerted to the frog's care prior to your purchase of it.
All food for thought.
You are doing a really great job- preparation/research PRIOR to obtaining a frog is too often overlooked!
Lynn