I'm assuming its about a 45cm long tank for it to be 40L. For your tank size you don't really want anything that exceeds about 5cm otherwise it'll become extra work for you
If so you can start with a lot of various bottom feeders. I personally would go for a group (4-7) corydoras. Probably something less boisterous like panda, trillieneatus (also known as julii), hastatus or even pygmus.
Otherwise ottos or small plecos (L104, or even a juvenile hypancistrus - l333, l066 etc).
For the rest of the tank a small to medium schooling tetra. Something like a Serpae or Rosy.
I hope that helps.