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[MrBredli]

The size is really up to you, it depends how you intend to set it up. Probably the most important thing to bear in mind however is filtration and drainage. You will need to have a good filtration system to keep the water clean. You will also need to be able to drain the water completely so you can give it a good scrub out every now and then. All things to keep in mind when you're drawing up the plans.