Until I started keeping fish, I believed all the rubbish that you read - that they only have an attention span of 20 seconds, that by the time they've gone round the tank once they've forgotten who they are and what they're doing, etc etc. This is complete and utter rubbish.
Some fish are clever. It's fair to say that none of my fish have yet to write a novel or direct a movie, but sometimes you get the distinct impression that they are looking at you. In a funny way.
I have two freshwater tanks, one full of fish from Lake Malawi (or Lake Nyassa), and one predominantly but not exclusively Amazonian. The Malawi tank is the most temperamental but the most rewarding: Lake Malawi, like many of the other East African rift valley lakes, is home to hundreds of species of fish which don't exist anywhere else. I keep varieties of Mbuna - rock dwelling cichlids which can vary from mildly to extremely aggressive. They are maternal mouthbrooders, which means that the females place the fertilised eggs in their mouths and keep them there for up to a month until the eggs have hatched and the fry (baby fish) are so big that they have to spit them out. Sounds a bit gross, but it is fascinating to watch.
I could go on for pages, but you may think it is a bit anoraky, so I will limit myself to a video - just click on the left.