I think there are quite a few things that can go in a 25l.
As Taffy Apple says, 8 or 10 of the very small shoaling fish (not zebra danios; avoid anything with horizontal stripes, as they're all fast swimmers and need more than 30cms space), but any of the microrasboras; dwarf emerald rasboras, chilli rasboras or CPD, or ember tetras, or a few male Endlers, or even a trio of sparkling gouramis or featherfin rainbows, or a betta, plus shrimps would be fine.
Don't forget you'll need to keep well on top of your water changes in a small tank like that.
How hard or soft is your water? That might narrow things down a bit and make it easier to decide.