Hello and welcome to FF! Sorry your frogs are not doing well . Feeder fish are vectors for parasites and other pathogens like bacteria. Great health risk to use with frogs. Suggest doing 25-50% weekly water changes versus 100%. Also; when you clean the gravel in your tank, it's killing the good bio-bacteria in it and your under gravel filter stops working until it cycles again in a month or so. Water changes should be conducted with same temperature water.

Recommend get some Melafix and Pimafix, then treat water with both daily for one week according to label instructions. You can prepare solution in gallon jugs or similar measuring device to try and figure out actual water volume in tank so there are no overdoses. Good luck !