I never use drain cleaners because of the chemicals and because you have a septic system. I'd try a bio type cleaner that uses enzymes and bacteria first. See if you can remove any hair that may be trapped just inside the drain. This may seem hokey but since you probably don't have snake take a wire hanger and untwist the ends. Use the twisted side as a makeshift snake but don't go too far just a little inside the tub drain to see if you can catch any hair. They also make a "Jet" type drain cleaner which uses pressure to clean out the pipes you can try that too, vinegar, coke, coffee, nair, I've heard of all these things being used with varying degrees of success but it's worth a try. I would use harsh chemicals as a last resort only.