Possibly too late, but the aquarium at London Zoo is, in my opinion, a better bet for enthusiasts than the London Aquarium. No walk-through shark tanks, but some truly lovely reef tanks, and some decent planted biotopes. If you are in South London, the Horniman Museum has an interesting small aquarium section - it's only £2 to get in, which reflects it's size, so I wouldn't travel across the whole of London to get to it, but it is nice. ADC is a decent fish shop, but Living Waters is an experience.