A tank that small would be a bit too small for most stockings, including some that I have seen in this thread. The options that I can see are a pair of small killies or maybe a small group of Heterandria formosa. Another option is the old stand by, the Betta splendens, called a fighter in the UK. For a clean up crew, you could maybe put 2 or 3 of the dwarf cories in that tank but it would start to limit what else you could keep. Another option, if you are willing to go with no females, is a few endlers. Beware that a single endler female would fill that tank in about a month or maybe less. They are nothing if not prolific.