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I thought i'd need to buy loads of stuff bit it seems like all i have to buy is a better tank (30 gallons) and a filter with a built in air supply feature.
Does anyone know how much these cost in the UK and which type of filter is best?
You can buy a 30 gallon tank fairly cheaply (around £25 for a basic tank without a hood) in one of the larger retail stores like Pets At Home. I would suggest something like a Fluval 4+ internal power filter (around £40) as this will give you good filtration for nowhere near the cost of a canister filter. It aerates and well as filters (it has a buil-in venturi device for surface agitation) and is fairly easy to run and maintain; I had two of them for years. As for a hood, if you want one with a light already built in that's probably going to be at least £50, I'm afraid (plus the tube itself - around £8). A plain one is much less, but of course you can't see your fish in the evenings!
It is definitely worth surfing around on ebay too as you can pick up some real bargains there very cheaply.
Hope this helps.

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