Yep, spring water should be fine since its clear of all the chlorine & such thats harmful.
I never recommend having a filter for anything under 5g, so you should be fine as long as you keep up with the water changes(which I know you will ).

And the decor is basically the same as if the ACF were a fish. Just make sure the plants are made of a material that won't scratch up the frog's skin.

Good luck! I'll be around if you have anymore questions.