i bought a tune 6string bass but now i found that if i don't loose the string after i play it a week later the neck will up bow. only a truss rod in my tune's neck.

some one told me it is not good to loosen the string or tightened it often.

can u give me some advice ?

It sounds like you may need to tighten the truss rod in your bass, but a trip to a qualified repair person is definitely a good idea. Good luck!