Water feature for a small vivarium?

[killerecho]

I need help implementing a waterfall/pond area in a small tank (10-20 gallon). I've combed through all the info I could find on this forum and others but have yet to find a clear, conclusive idea. I've tried a number of methods and failed, with the water quickly becoming stagnant and polluted or leaking into areas it didn't belong, flooding the substrate.

The vivs I want to add water areas to have false bottom hydroton layers and live plants. They will house gray tree frogs and pacific chorus frogs, which will presumably dip in the water nightly (and defecating copiously). A waterfall would be great but not absolutely necessary. I would just like a shallow pool - 1-1.5 inch deep. I would prefer not to drill holes in the glass if possible.

I guess it boils down to 3 questions: what's the best way to do this; how often does the water need changing; and what equipment do I need?

Thanks for any advice you can share.