That's lovely. I think it looks great with just the plants but I can certainly see a shrimp or two running around it. The plants will make keeping it easier too.
You could hunt around for some rush type plants, they are usually marginal plants and like bog. Unfortunately all the bog/ marginal plants that spring to mind right now are quite large and more suited to a pond. But their is bound to be something suitable.
I will have a hunt around and see what I can find.
ok the first plant that I think would be suitable and give you a third dimension to the bowl is Low Bulrush Scirpus cernuus (growth 8inches/ 20cm) which may still be too high. Also Fairy Lily Zephyranthus candida, but this will like it a bit dryer and it does tend to multiply bulbs like crazy, which is fine if you have a garden you can plant extra bulbs into.
No worries, I think the bowl will make an amazing micro scape
following Ikebana and Takashi Amano principals.
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If the rushes idea doesn't work you could even get an interesting piece of branch/ wood and stand it up then drap something like old mans beard over the twigs or even attach Tilandsia's to the piece of interesting branch. The posibilities are almost endless.