What is the name of the shop u bought the tank from?
Under uk law, you have no legal right to a refund for an unwanted product. If you have used the tank, you have no chance of recieving a refund. If the tank is not used, you should take it back to the shop and ask for a refund, or ask to exchange it for the filters and lights that you need. As a last resort, you could ask for a credit note to use later on. Alot of shops will be reasonable enough to do one of the above, as they will most importantly to them, want to keep your custom.