African Clawed frogs are very efficient animals and hold on to body fat (around the kidneys) very well. Which is why they can survive starvation conditions for a very long time.

As far as earthworms, I feed mine nightcrawlers often. Even my under 2" male frog gets a full nightcrawler now and then and he does just fine with it. If your frogs are becoming obese (which I doubt because a growing frog will turn energy into growth) then feed less often.

I would alternate between reptomin and nightcrawlers, toss in crickets once every few weeks for a live treat. I find throwing live crickets into the water is very stimulating for these frogs, they enjoy hunting.