Simple problem here. The bullfrog has roughed up his face on the sides of his enclosure. A twenty-long is too small for a typical Ranid like Lithobates catesbeianus. Most Ranids are skittish and are startled easily. If kept in a tank that is too small, like a twenty-gallon-long, they will injure themselves by launching themselves into the sides of the tank when startled. So my advice to you is to get a much larger enclosre (55 or larger) and have a vet take a look at its injuries.

Also, drop the goldfish for the frogs diet, as they are not a healthy item for them to eat. Stick with various kinds of insects and earthworms.