Well, with a 180L tank, you can get most commonly found tropical species. I'm assuming you want tropical species, by the way. If you don't want to be messing around too much with water parameters then some African Cichlids tend to prefer moderately hard water, plus you get a variety of colours and species and their interactions can be very interesting. I know someone who used to keep African Cichlids, I can't remember if they were Mbuna or Malawi, but they were very happy. The one thing I would say, is they have a tendency to be territorial, even aggressive, so I've often seen it advised to keep a lot of them to make sure no one fish gets massively bullied. Someone who has kept them before could probably give you more information. Hope this helps.