The licence is free, it lasts for 1 year.
£50 is just a fee to take them from the aircraft to your car - you have to pay for the fish, the freight charges (which vary from airline to airline, number of boxes etc etc) - on 1 box freight is likely to cost in the region of £40 to £50, maybe less depending upon weight and airline. In addition you will have veterinary inspection fees in country of origin - this is likely to cost (in the states) £10 (? something like that), handling and box charges - depending upon who boxes up the fish, and where they have to be taken too, then you have VAT at the UK end - which obviously depends on the value of the fish - plus £25 for Defra inspection charges - photocopying and administration - so, what can appear to be cheap fish - cost an absolute fortune to deliver to the UK.
You won't be able to buy fish in the states and take them to the airport to check in, airlines won't take live baggage - you'll have to arrange for the fish to be sent as frieght - which involves the above steps -
get the tank, decide what you want, where from and see if its going to be worth importing ...
any other Q's just ask
carl