since saltwater would be out of the budget how bout 100 of different community fish with a little key for each species giving its decription and location, ect
im talking guppies, tetras, dwarf puffers, dwarf chichlids or angels, tiny cats small plecos, anything that doesnt get bigger than say 6 or 7 inches. think a couple big schools of neons or other tertas and barbs and maybe a couple of the big non aggressive cichlids as a center piece. different inverts you got that island you could make it into a land type thing for things like freshwater crabs do something different than big fish . i wanna see a huge tank or pond with a bunch of different community fish. kids would love it
since its like a window and if you can get enough sunlight in get a lot of different plants for it. and for that tree maybe keep something right there that likes tree but is afraid of water or something