Hey there,

Yes green and golden bell frogs can grow quite large. Males grow to around 8cm and females about 11cm.

I am unsure of gallons (I prefer to go by dimensions when talking about frogs) but roughly speaking two bell frogs should have a MINIMUM of 60x30x30cm (LxWxH) for their tank and have about 1/3 of that space as water and the rest land. But as with most frogs the bigger the tank the better.

What are the measurements of your 20 gallon?