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UV Sterilizers are sold with ratings up to a certain gallon amount. Coralife Turbo Twist UV Sterilizerare a good choice. The "6X" Model (18w) is rated for 250 Gallons, and the "12X" Model (36w) is rated for up to 500 Gallons. Choose one or the other, or you could use a pond sterilizer, which are rated for large bodies of water.
UVB Lights, should cover all or almost all of the enclosure/tank. So at minimum a 4 foot long fixture, or two three foot long sections, to cover the full length. And since a 300 gallon tank is wide from front to back, it would be best to have two rows of lights. Remember the UV light cannot pass through glass or acrylic or it will be useless.