First off what kind of tank is it? Plastic? I'm asking because of tanks cannot hold their weight with water, I would do a test first and fill it all the way up with water and check on it a day later, if you notice some cracks developing it's unsafe to house a fish in it. Although some plastic tanks can hold their weight with water, like Lee's Kritter Keepers.
Remember to not wash with soap, it'll leave a residue that's toxic to fish. You can wash the tank with hot water and vinegar. One part vinegar for every 9 parts of water. That'll get rid of anything nasty in there that's lurking. Also what kind of light? Like kikitchi asked, if it's a heat lamp I wouldn't recommend on using it as it may overheat the water, Bettas like tropical waters but really hot water will turn them into soup!
Hope that helps. :)