I'm not a collector, but AquaLogic on Brook Drive (near Elepahnt/Imperial War Museum has lots of variety. www.aqualogic.com. No connection, honestly! Just enjoyed looking in on my way back from a boozy weekend lunch at Bar Room Bar.
You can get excellent aquarium DVD's from HMV, Virgin etc; with great sound effects.
And you can turn up the volume for the soothing sound of water.
No maintenance, and no worries when you are away. And a much greater variety of fish on display
than you could hope to have in a domestic tank. Lionfish, seahorses and suchlike.