Here's a link to the beginner's guide to cycling
. It will probably be able to explain things much better than I can!
Basically; fish, food, and plant wastes create ammonia - which is toxic to fish/frogs
A bacteria forms that converts the ammonia into nitrItes - which is ALSO toxic to fish/frogs
A bacteria THEN forms that converts the nitrItes into nitrAtes - which is NOT toxic to fish/frogs until it reaches VERY high levels.
In a fully cycled tank you should see:
Ammonia: 0 ppm
NitrIte: 0 ppm
NitrAte: usually between 7.5-20ppm (this TOTALLY depends on tank size/stocking)
As this cycle establishes in your tank, it is very unsafe for the creatures that live there. First the ammonia will 'spike' as the levels of wastes in the tank rise with nothing to stop it (be sure to do plenty of water changes to keep this number down for the frogs). When the next stage of the cycle begins, the ammonia will start to vanish, but the nitrIte levels will spike (be sure to do plenty of water changes to keep this number down for the frogs) Finally, at the last stage, the nitrAte levels will spike as the nitrIte levels fall. . . (again with the water changes), and finally, the nitrAtes will settle where they need to be to keep a balance in your tank, giving you a 0 reading on ammonia and nitrIte - and a low reading of nitrAtes.
Live plants WILL and DO help with keeping things in check during cycling. They absorb the poisonous toxins from the water and convert it into their own food - so yay for live plants! But you'd need to have a LOT of live plants to absorb all of the nitrate in a fully cycled tank.
There are many people around who can explain this stuff far better than I can! Check out that link, and hopefully one of them will pop by to help out soon! In the meantime, keep testing and if your levels start to rise, keep them down with water changes.
Out of curiosity, what size is the tank and how long has it been set up? Many people choose not to cycle smaller tanks, and prefer doing complete water changes every so often. This isn't my way, though, so maybe someone can help you there if that's the way you want to go.
Good call on adding Frank later, hopefully they'll get along swimmingly! And I'm sure you'll be able to tell your froggies apart soon! Can't wait to hear what names they get!