I have set up a tank for my betta?

your tank sounds great...just make sure the pebbles from the river were really, really clean and sterilized before you added them. 16 litres is fine, and you have a heater and filter which is also good...so many people don't think is necessary, but it is vitally important for a healthy fish. There are a few tropical species whcih are compatible with bettas, but your tank isn't really big enough for a community...i would recommend lots of research first as some fish will nip at your bettas fins and damage them, and really stress him out. Another option is of course getting two or three females, but the males are just so gorgeous so I would recommend a male over a female....their so gorgeous, anything else in the tank would be overlooked anyway, so just get a male Betta and let him enjoy his lovely tank and have it all to himself. Good luck =)