For a while when offering food to your frog just drop the food items from the tongs so that the frog can see you dropping the food from the tongs. Eventually after seeing that foos comes from the tongs they will make the connection and will take from them eventually.
You can also gently rub the food item on the frog's mouth and some will become annoyed enough to bite the food thus taking it from the tongs.
Now not all frogs will learn in these ways, but many will. Also not all frogs will actually accept food from tongs ever. This just depends on the frog's temperment and personality.
Philippe De Vosjoli and Robert Mailloux do say a 20 gallon is ok, BUT in reality you need a smaller enclosure. You can seal off half or more of the tank until your frog is large enough. A 20 gallon tank is quite large and will also be slightly more difficult to heat and maintain the climate in so it may be best to block off half or more of the tank so that your lityke frog does not become stressed by all that space. They also won't use most of the enclosure so it will be overkill and pretty much useless.