A 40l (~45cm long) tank is going to really limit your options for any fish species, yet alone loaches, for their long term needs. The Zebra Loaches, for example, would need something like a 120cm long tank long term, as they should reach ~10cm and are quite active fish.
Besides the Rosy Loaches I mentioned earlier (that come from the same area of Celestial Pearl Danios, a small group of each making a great Myanmar biotope in your tank), I'm struggling for ideas besides some of the smaller "Hillstream Loach," but then most of these fish need a mature tank with algae growth and associated organisms. The carnivores species like Aborichthys ("Squirrel Loaches") genus might work, but they are not easy to get hold of.