That is what the basic idea is that I have. I don't know about wasted air space but I definitely want something under the plant substrate to prevent bogging.

Pond will likely be as large as I can make it or as large a container as I can put in there. Hoping to find a deep glass dish that will fit right in there so less need to fill in gaps and stuff to keep both food and frogs out. Will be less than half though, more likely 1/3 to 1/4 of the length and full depth of the tank. I don't want to silicone a piece of glass in but I may just do it to make it simpler. Can always take it out if I want to change things another day.