20 gallon will be very nice for a pair of D. Tinctorius (Azurius, Powder Blue, Cobalt - note that if you are thinking about getting azureus, you shouldn't put two females together as they are very territorial, but that's not what you are aiming for).

Some of our members here are very happy with D. Leucomelas. These do really well in pairs and groups.

Another beginner dart frogs are the D. auratus. As opposed to the tincts and leucs, these are semi-arboreal.... therefore you would need to think about building a viv that will have lots of places to climb (tincts and leucs will also be caught climbing, but they are mostly terrestrial frogs).

These are just a few choices you may want to consider... There are lots more to chose, but it's a good starting point.

When you build your viv, you'll have to think about keeping it very humid, and make sure it stays at least at room temperature. Seeding springtails in advance will also be beneficial for the viv as the frogs will feed on this microfauna... and the springtails will also be playing the roles of janitors in your viv, eating feces and dead leaves, etc.

Have you thought about culturing fruit flies? It will be your frogs main source of food.... Something to consider...

Keep us posted!

Eric