Ah, the Walstad method, giant-sized.
I say you should just go shrimp, either that or solitary fish, like a Betta. With 25g and no mechanical filtration you have limited options for stocking. Unless you maintain diligence in water changes and have nitrogen-hogging plants, you wouldn't be able to keep anything larger than a small school in there at optimal parameters. Of course you can always get away with letting things slide, as fish are highly adaptable... I wouldn't recommend it though.
Shoot for small bioload stock, and overkill the waste with fast growing plants, what you have now is not enough to filter a 25g for stock. You have Duckweed, but Ricca looks nicer and isn't a huge PITA when it comes to rescaping your tank or removing it entirely.
Many of the mosses are great for this kind of tank, they grow fast (under higher PAR), filter pretty well, and are pretty nice looking. Taiwan, Java, Xmas, are all good fast growing mosses.
I like the glowing tetras for set ups like this, my go to is the Cardinal, you can easily keep a school of 10-15 in there with no problems. CPD are always a nice choice, but that depends on what kind of fish you want.
I would go with loaches or other bottom dwelling fish that don't require a lot of water movement or light.
But before anything, get more plants in there! It is not densely planted enough for stock!
Last edited by AVN; 11-25-2012 at 10:02 PM.