I am cycling my first tank, a 35 litre. I decided to cycle with fish, but now kinda wish I hadn't. I bought 3 leopard danios, but one has already died and now one of the other ones is not looking so well. If I had the chance again I would definately do a fishless cycle so that I could get the water perfect beforehand. I guess I didn't research enough before I started and used the instructions I got with the aquarium, which told me to add fish after 24 hours. It's all a learning curve though! I'll never make the same mistake again!