Tarantula pet environment help?

I have a Brazilian Red and White Tarantula, it stays in 5 gallon fish tank and has all its necessary requirements. There's is one thing that I want to know though, I have got a sandy substrate in the environment and I live in an area with a relatively high humidity. Is this substrate ok to use if I just slightly mist the sand with a spray mister, or do I need to get potting soil?