Interesting, at 25L/6.6USG that thing must be bigger than it looks. A biorb competitor! You could have a nice little school of 6 neon tetras in there with some plants. With an orb shape you have less surface area to provide oxygen for fish stock, so the 1" of mature fish body per US gallon would be pushing it but should be ok if you do a good fishless cycle, keep your media clean and are frequent with your water changes I guess.
~~waterdrop~~