I believe some people feed them guppies and sooner or later they are going to attempt to make a meal out of your fish.
Veggiegirl, You're thinking of African Clawed
Frogs. African Dwarf frogs stay small. Females max out at an inch and a half (3.81 cm), and males are smaller. I have female guppies that are longer than my ADFs stretched out, but those females are bite sized for my ACFs.
A 2.5 is cramped for just the two frogs, a five gallon is recommended (by other Dwarf frog keepers/breeders) for up to 5 adult frogs, with appropriate water changes, and filtration.
The 2.5 would be suitable for your betta, and not much else. There are too many fish labeled as "algae eaters" for me to help with how much space it should have, or whether it should be housed with other members of its species.
Your frogs may be more territorial than most, which would explain why they were bothering the Algae eater, and lunging at your betta.
OR, you were actually sold ACFs instead of ADFs, which can, and does happen!
Do they have webbed fingers on all four feet?