If your looking for something more reminiscent of your ponds and lakes, leopard frogs are a good choice. They are easy to care for and two would be happy in a 20 gallon. If you want something smaller, fire bellied toads are super easy to care for and three can fit in a ten gallon tank comfortably. Stay away from bullfrogs as they need a 100 gallon plus enclosure for just one. Other options for tanks 30 gallons or under include northern green frogs, pickerel frogs, common frogs. Just a few examples.