chlorhexadine AKA nolvasan Is the best bang for your buck, If you want to be safe that is. it only takes one to two squirts per tub and it will disinfect it. Vets use it, Its perfectly harmless you could drink it and be fine... Although I suggest you don't. And its organic... Check it.

http://www.petvetsupply.com/dismfra001.html You can find it cheaper at feed stores like tractor supply.