I'm glad that you're not going down the 'nemo' route, I've had four different reef systems in recent years and they've all been fairly hard work. I've already wasted a lot of money due to my mistakes in the learning process.

For a 23 litre tank I would suggest a single Siamese fighting fish (Betta splendens) or if you can find them, Microrasboras, these are absolutely wonderful little fish.